depends: enable pre-compiled headers for qt

All trusty compilers work ok with this now, and it shaves a few minutes off of
build time

depends/packages/qt.mk

@@ -31,7 +31,7 @@ $(package)_config_opts += -no-iconv
 $(package)_config_opts += -no-gif
 $(package)_config_opts += -no-freetype
 $(package)_config_opts += -no-nis
-$(package)_config_opts += -no-pch
+$(package)_config_opts += -pch
 $(package)_config_opts += -no-qml-debug
 $(package)_config_opts += -nomake examples
 $(package)_config_opts += -nomake tests