Set QMAKE_CFLAGS_RELEASE_WITH_DEBUGINFO for win32-g++

The win32-g++ mkspec is not based on gcc-base, so
QMAKE_CFLAGS_RELEASE_WITH_DEBUGINFO is not inherited. Therefore,
-release -force-debug-info would build with neither -O2 nor -g.

Change-Id: I4e97cb08f577062dd342fb3e91c02adfd636a310
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

mkspecs/win32-g++/qmake.conf

@@ -31,6 +31,7 @@ QMAKE_CFLAGS_DEPS       = -M
 QMAKE_CFLAGS_WARN_ON    = -Wall -Wextra
 QMAKE_CFLAGS_WARN_OFF   = -w
 QMAKE_CFLAGS_RELEASE    = -O2
+QMAKE_CFLAGS_RELEASE_WITH_DEBUGINFO = -O2 -g
 QMAKE_CFLAGS_DEBUG      = -g
 QMAKE_CFLAGS_YACC       = -Wno-unused -Wno-parentheses
 QMAKE_CFLAGS_SPLIT_SECTIONS = -ffunction-sections