qt: disable precompiled headers to workaround compilation failure

Precompiled header implementation is buggy in current Qt as described at
http://comments.gmane.org/gmane.comp.lib.qt.devel/5933.

The only known workaround is currently to add -no-pch to configure.

Fixes build failure terminating with error
  compiling ../3rdparty/pixman/pixman-arm-neon-asm.S
  cc1: fatal error: .pch/release-shared-emb-arm/QtGui: No such file or directory
  compilation terminated.
such as:
http://autobuild.buildroot.net/results/648265012591280fa592a239f86ee39aec2d5fa6/build-end.log

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/qt/qt.mk

@@ -416,10 +416,10 @@ else
 QT_CONFIGURE_OPTS += -no-declarative
 endif
 
-# ccache and precompiled headers don't play well together
-ifeq ($(BR2_CCACHE),y)
+# -no-pch is needed to workaround the issue described at
+# http://comments.gmane.org/gmane.comp.lib.qt.devel/5933.
+# In addition, ccache and precompiled headers don't play well together
 QT_CONFIGURE_OPTS += -no-pch
-endif
 
 # x86x86fix
 # Workaround Qt Embedded bug when crosscompiling for x86 under x86 with linux