CMake: Fix FindRT warning

This fixes the following warning:
The package name passed to `find_package_handle_standard_args` (rt) does                                                                                                                                                                   not match the name of the calling package (RT).

cmake-modules/FindRT.cmake

@@ -16,5 +16,5 @@ set(RT_LIBRARIES ${RT_LIBRARY})
 # handle the QUIETLY and REQUIRED arguments and set
 # RT_FOUND to TRUE if all listed variables are TRUE
 include(FindPackageHandleStandardArgs)
-find_package_handle_standard_args(rt DEFAULT_MSG RT_LIBRARY)
+find_package_handle_standard_args(RT DEFAULT_MSG RT_LIBRARY)
 mark_as_advanced(RT_LIBRARY)