Fix MYSQL_MAINTAINER_MODE=ERR, on Windows, with Ninja , in 10.2

A conversion warning 4267 that we want to disable(prior to 10.3),
was suppressed with  cmake VS generator for C++ and C, despite being set
only for CXX flags.

The fix is to disable the warning in C flags, too. In 10.2, this warning
is noisy, in 10.3 it is fixed.

cmake/os/Windows.cmake

@@ -150,6 +150,7 @@ IF(MSVC)
   IF(CMAKE_SIZEOF_VOID_P EQUAL 8)
     # Temporarily disable size_t warnings, due to their amount
     SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /wd4267")
+    SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /wd4267")
   ENDIF()
   IF(MYSQL_MAINTAINER_MODE MATCHES "ERR")
     SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /WX")