Replace win32-g++ with mingw scope

Commit 773dd01 in qtbase introduced a general mingw platform scope, which
is cleaner and more flexible than matching the spec name.

(see 278152fff for a similar commit in qtbase).

Change-Id: I430ab289af99e96cd6ed6edfd683258defcb13a4
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

src/svg/svg.pro

@@ -45,7 +45,7 @@ wince*: {
 }
 
 contains(QT_CONFIG, system-zlib) {
-    if(unix|win32-g++*):     LIBS_PRIVATE += -lz
+    if(unix|mingw):          LIBS_PRIVATE += -lz
     else:                    LIBS += zdll.lib
 } else {
     git_build: \