Missing PRIVATE in cmake. (#539)

CMakeLists.txt

@@ -299,7 +299,7 @@ if(NOT SNMALLOC_HEADER_ONLY_LIBRARY)
       if(SNMALLOC_OPTIMISE_FOR_CURRENT_MACHINE)
         check_cxx_compiler_flag(-march=native SUPPORT_MARCH_NATIVE)
         if (SUPPORT_MARCH_NATIVE)
-          target_compile_options(${name} -march=native)
+          target_compile_options(${name} PRIVATE -march=native)
         else()
           message(WARNING "Compiler does not support `-march=native` required by SNMALLOC_OPTIMISE_FOR_CURRENT_MACHINE")
         endif()