In addition to r260102, also define GCC_MS_EXTENSIONS in bsd.sys.mk,

since kernel module builds do not use kern.pre.mk.

MFC after:	3 days
X-MFC-With:	r260102

share/mk/bsd.sys.mk

@@ -123,6 +123,7 @@ CFLAGS+=	 -Qunused-arguments
 CFLAGS+=	 ${CFLAGS.clang}
 CXXFLAGS+=	 ${CXXFLAGS.clang}
 .else # !CLANG
+GCC_MS_EXTENSIONS= -fms-extensions
 CFLAGS+=	 ${CFLAGS.gcc}
 CXXFLAGS+=	 ${CXXFLAGS.gcc}
 .endif # CLANG