CMake: StringProcessing: Set C visibility hidden

Don't expose the C symbols from StringProcessing.

Author: etcwilde

Runtimes/Supplemental/StringProcessing/CMakeLists.txt

@@ -24,6 +24,9 @@ if(NOT PROJECT_IS_TOP_LEVEL)
   message(FATAL_ERROR "Swift StringProcessing must build as a standalone project")
 endif()
 
+set(CMAKE_C_VISIBILITY_PRESET "hidden")
+set(CMAKE_VISIBILITY_INLINES_HIDDEN YES)
+
 set(CMAKE_POSITION_INDEPENDENT_CODE YES)
 
 set(${PROJECT_NAME}_SWIFTC_SOURCE_DIR