add check for FTBFS condition

only happened once so far on ppc64le, but let's be sure

qt5-qtwebkit.spec

@@ -131,6 +131,10 @@ BuildArch: noarch
 # find/fix pngs with "libpng warning: iCCP: known incorrect sRGB profile"
 find -name \*.png | xargs -n3 pngcrush -ow -fix
 
+# ppc64le failed once with
+# make[2]: *** No rule to make target 'Source/WebCore/Resources/textAreaResizeCorner.png', needed by 'Source/WebKit/qrc_WebCore.cpp'.  Stop.
+test -f Source/WebCore/Resources/textAreaResizeCorner.png
+
 
 %build
 # The following changes of optflags ietc. are adapted from webkitgtk4 package, which