Add mkspecs include path to the gn build

INCLUDEPATH qmake variable doesn't contain mkspecs include path. Add it
explicitly to the include_dirs gn variable in the gn generator.

Change-Id: I454aa015794f258f588590b4662a16c04041c8fd
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>

mkspecs/features/gn_generator.prf

@@ -175,7 +175,8 @@ GN_CONTENTS += "  if (!defined(include_dirs)) {"\
                "  }"
 GN_CONTENTS += "  include_dirs += ["
 for (path, INCLUDEPATH): GN_CONTENTS += "    \"$$path\","
-GN_CONTENTS += "    rebase_path(\"$target_gen_dir/.moc/\")"
+GN_CONTENTS += "    rebase_path(\"$target_gen_dir/.moc/\"),"
+GN_CONTENTS += "    \"$$QMAKESPEC\""
 GN_CONTENTS += "  ]"
 
 GN_CONTENTS += "  if (!defined(ldflags)) {"\