Indirect dependency on boost_headers

...since 2eb36dc4 "Hack to not leave SolarMutex
released after g_main_loop_run call."

Change-Id: Ib4eb92002531af423b3ed4f24f6f4b71dc9cdc0d

ucb/Library_ucpgio1.mk

@@ -12,6 +12,8 @@ $(eval $(call gb_Library_Library,ucpgio1))
 
 $(eval $(call gb_Library_set_componentfile,ucpgio1,ucb/source/ucp/gio/ucpgio))
 
+$(eval $(call gb_Library_use_external,ucpgio1,boost_headers))
+
 $(eval $(call gb_Library_use_sdk_api,ucpgio1))
 
 $(eval $(call gb_Library_use_libraries,ucpgio1,\