CMakeRCCompiler: Handle uppercase 'RC' as resource file format extension

We already match lowercase 'rc'.  Add 'RC' too.  The latter is listed in
other CMake<LANG>Compiler modules as ignored for their languages already.
Signed-off-by: Tim Blechmann <tim@klingt.org>

Modules/CMakeRCCompiler.cmake.in

 set(CMAKE_RC_COMPILER "@CMAKE_RC_COMPILER@")
 set(CMAKE_RC_COMPILER_ARG1 "@CMAKE_RC_COMPILER_ARG1@")
 set(CMAKE_RC_COMPILER_LOADED 1)
-set(CMAKE_RC_SOURCE_FILE_EXTENSIONS rc)
+set(CMAKE_RC_SOURCE_FILE_EXTENSIONS rc;RC)
 set(CMAKE_RC_OUTPUT_EXTENSION @CMAKE_RC_OUTPUT_EXTENSION@)
 set(CMAKE_RC_COMPILER_ENV_VAR "RC")