winbuild: Define CARES_STATICLIB when WITH_CARES=static

When libcurl is built with MODE=static, c-ares is forced into static
linkage too. That doesn't happen when MODE=dll so linker would break
over undefined symbols.

closes https://github.com/curl/curl/pull/4688

winbuild/MakefileBuild.vc

@@ -222,6 +222,9 @@ CARES          = static
 
 !IFDEF USE_CARES
 CARES_CFLAGS   = /DUSE_ARES /I"$(CARES_INC_DIR)"
+!IF "$(CARES)"=="static"
+CARES_CFLAGS   = $(CARES_CFLAGS) /DCARES_STATICLIB
+!ENDIF
 !ENDIF