KNOWN_BUGS: cmake build in Linux links libcurl to libdl

Closes #6165

docs/KNOWN_BUGS

@@ -121,10 +121,11 @@ problems may have been fixed or changed somewhat since this was written!
  14.1 DICT responses show the underlying protocol
 
  15. CMake
- 15.1 use correct SONAME for shared library
+ 15.1 use correct SONAME
  15.2 support build with GnuTLS
  15.3 unusable tool_hugehelp.c with MinGW
  15.4 build docs/curl.1
+ 15.5 build on Linux links libcurl to libdl
 
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@@ -836,7 +837,7 @@ problems may have been fixed or changed somewhat since this was written!
 
 15. CMake
 
-15.1 use correct SONAME for shared library
+15.1 use correct SONAME
 
  The autotools build sets the SONAME properly according to VERSIONINFO in
  lib/Makefile.am and so should cmake to make comparable build.
@@ -854,3 +855,9 @@ problems may have been fixed or changed somewhat since this was written!
  The cmake build doesn't create the docs/curl.1 file and therefor must rely on
  it being there already. This makes the --manual option not work and test
  cases like 1139 can't function.
+
+15.5 build on Linux links libcurl to libdl
+
+ ... which it shouldn't need to!
+
+ See https://github.com/curl/curl/issues/6165