Code cleanup - Xcode 9 build warning/issue.

Summary:
Xcode 9 has compiler settings that are more strict.  This can occur if someone updates there project to use the default settings.

This patch declares the default type instead of allowing the compiler to determine it.  Instead of `()` we now say `(void)` in a block call.

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It was just annoying me, and it has no side effects.  If there are side effects, then we should fix the type and not go with empty to represent void.

Update project settings in Xcode.  This code doesn't have any known side effects since the compiler assumes the type is void when not declared.

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  CATEGORY
[----------]        TYPE
[ CLI      ]   [-------------]      LOCATION
[ DOCS     ]   [ BREAKING    ]   [-------------]
[ GENERAl  ]   [ BUGFIX      ]   [-{Component}-]
[ INTERNAL ]   [ ENHANCEMENT ]   [ {File}      ]
[ IOS      ]   [ FEATURE     ]   [ {Directory} ]   |-----------|
[ ANDROID  ]   [ MINOR       ]   [ {Framework} ] - | {Message} |
[----------]   [-------------]   [-------------]   |-----------|

[CATEGORY] [TYPE] [LOCATION] - MESSAGE

 EXAMPLES:

 [IOS] [BREAKING] [FlatList] - Change a thing that breaks other things
 [ANDROID] [BUGFIX] [TextInput] - Did a thing to TextInput
 [CLI] [FEATURE] [local-cli/info/info.js] - CLI easier to do things with
 [DOCS] [BUGFIX] [GettingStarted.md] - Accidentally a thing/word
 [GENERAL] [ENHANCEMENT] [Yoga] - Added new yoga thing/position
 [INTERNAL] [FEATURE] [./scripts] - Added thing to script that nobody will see
-->
[DOCS] - Fixed potential compiler build issue on Xcode 9 after updating settings in project.
Closes https://github.com/facebook/react-native/pull/16554

Differential Revision: D6184949

Pulled By: shergin

fbshipit-source-id: 23083248a39c56f5cf50b5ff4390629dd6335f84

Libraries/Image/RCTImageStoreManager.h

@@ -11,7 +11,7 @@
  * Set and get cached image data asynchronously. It is safe to call these from any
  * thread. The callbacks will be called on an unspecified thread.
  */
-- (void)removeImageForTag:(NSString *)imageTag withBlock:(void (^)())block;
+- (void)removeImageForTag:(NSString *)imageTag withBlock:(void (^)(void))block;
 - (void)storeImageData:(NSData *)imageData withBlock:(void (^)(NSString *imageTag))block;
 - (void)getImageDataForTag:(NSString *)imageTag withBlock:(void (^)(NSData *imageData))block;
 

Libraries/Image/RCTImageStoreManager.m

@@ -37,7 +37,7 @@ RCT_EXPORT_MODULE()
     return 1;
 }
 
-- (void)removeImageForTag:(NSString *)imageTag withBlock:(void (^)())block
+- (void)removeImageForTag:(NSString *)imageTag withBlock:(void (^)(void))block
 {
   dispatch_async(_methodQueue, ^{
     [self removeImageForTag:imageTag];