Moved suggest-override to the GCC only section

I did not realise Clang 6 was the earliest supported Clang version.

cmake/CompilerWarnings.cmake

@@ -56,7 +56,6 @@ function(
         -Wdouble-promotion # warn if float is implicit promoted to double
         -Wformat=2 # warn on security issues around functions that format output (ie printf)
         -Wimplicit-fallthrough # warn on statements that fallthrough without an explicit annotation
-        -Wsuggest-override # warn if an overridden member function is not marked 'override' or 'final'
     )
   endif()
 
@@ -68,6 +67,7 @@ function(
         -Wduplicated-branches # warn if if / else branches have duplicated code
         -Wlogical-op # warn about logical operations being used where bitwise were probably wanted
         -Wuseless-cast # warn if you perform a cast to the same type
+        -Wsuggest-override # warn if an overridden member function is not marked 'override' or 'final'
     )
   endif()