ruby3_rstring_getmem: suppres warning

icc warns at this line.

> include/ruby/3/core/rstring.h(126): warning #413: variable "retval" has an uninitialized const field
>           struct RString retval;
>                          ^

This must not be a problem because uninitialized const field is not used
at all.  But the warnings are annoying.  Let us suppress them.

include/ruby/3/core/rstring.h

@@ -29,6 +29,7 @@
 #include "ruby/3/dllexport.h"
 #include "ruby/3/fl_type.h"
 #include "ruby/3/value_type.h"
+#include "ruby/3/warning_push.h"
 #include "ruby/assert.h"
 
 #define RSTRING(obj)            RUBY3_CAST((struct RString *)(obj))
@@ -111,6 +112,11 @@ RSTRING_EMBED_LEN(VALUE str)
     return RUBY3_CAST((long)f);
 }
 
+RUBY3_WARNING_PUSH()
+#if RUBY3_COMPILER_IS(Intel)
+RUBY3_WARNING_IGNORED(413)
+#endif
+
 RUBY3_ATTR_PURE_ON_NDEBUG()
 RUBY3_ATTR_ARTIFICIAL()
 static inline struct RString
@@ -130,6 +136,8 @@ ruby3_rstring_getmem(VALUE str)
     }
 }
 
+RUBY3_WARNING_POP()
+
 RUBY3_ATTR_PURE_ON_NDEBUG()
 RUBY3_ATTR_ARTIFICIAL()
 static inline long