build: patch qt libpng to fix powerpc build

This is an alternative to #19751 that fixes the build without requiring
splitting out libpng. This patch can be dropped once we are building qt
5.12.0 or later.

depends/patches/qt/fix_powerpc_libpng.patch

@@ -0,0 +1,23 @@
+commit 6f9feb773a43c5abfa3455da2e324180e789285b
+Author: fanquake <fanquake@gmail.com>
+Date:   Tue Sep 15 21:44:31 2020 +0800
+
+    Fix PowerPC build of libpng
+
+    See https://bugreports.qt.io/browse/QTBUG-66388.
+
+    Can be dropped when we are building qt 5.12.0 or later.
+
+diff --git a/qtbase/src/3rdparty/libpng/libpng.pro b/qtbase/src/3rdparty/libpng/libpng.pro
+index 577b61d8..a2f56669 100644
+--- a/qtbase/src/3rdparty/libpng/libpng.pro
++++ b/qtbase/src/3rdparty/libpng/libpng.pro
+@@ -10,7 +10,7 @@ MODULE_INCLUDEPATH = $$PWD
+ 
+ load(qt_helper_lib)
+ 
+-DEFINES += PNG_ARM_NEON_OPT=0
++DEFINES += PNG_ARM_NEON_OPT=0 PNG_POWERPC_VSX_OPT=0
+ SOURCES += \
+     png.c \
+     pngerror.c \