build: Sync gwt-debug target with install-dev target

This patch ensures that both gwt-debug and install-dev
targets use same GWT-specific defaults, as defined by
DEV_EXTRA_BUILD_FLAGS_GWT_DEFAULTS option.

This relates mainly to "gwt.userAgent" property, whose
value should be in sync for both targets mentione above.

Change-Id: I33760b721f78ab26eeefc1ef3bd662829c49c312
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>