CheckSymbolExists: Restore newline at end of test source

Refactoring in commit db76876db5 (Modules: Use new SOURCES_FROM_*
try_compile (1/2), 2022-09-26, v3.25.0-rc1~74^2~1) accidentally dropped
the final newline of the test source, which was previously provided by
the input to `configure_file`.  The C standard requires a newline at the
end of file, so add one explicitly.

Signed-off-by: Andrey Vostrikov <[email protected]>

Modules/CheckSymbolExists.cmake

@@ -136,7 +136,7 @@ int main(int argc, char** argv)
   ${_CSE_CHECK_NON_MACRO}")
     endif()
     string(APPEND _CSE_SOURCE "
-}")
+}\n")
     unset(_CSE_CHECK_NON_MACRO)
 
     if(NOT CMAKE_REQUIRED_QUIET)