headers: Define `__MINGW_ATTRIB_DEPRECATED_SEC_WARN` properly for C

When `__MINGW_MSVC_COMPAT_WARNINGS` is defined, the plain (non-template)
functions are always deprecated, and template overloads are not to be marked
with `__MINGW_ATTRIB_DEPRECATED_SEC_WARN` anyway.

Previously these warnings could not be disabled for C.

Reference: https://sourceforge.net/p/mingw-w64/mailman/message/59152860/
Signed-off-by: LIU Hao <lh_mouse@126.com>