enable ICU_HEADER includes for starmath

Although $include <i18nutils/unicode.hxx> compiles on Linux,
it doesn't compile on the Mac without this.

Change-Id: I56eae77b1c293b2348ca1fd7b38b543ea713557d
Reviewed-on: https://gerrit.libreoffice.org/19355Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Justin Luth <justin_luth@sil.org>
Tested-by: Justin Luth <justin_luth@sil.org>

starmath/Library_sm.mk

@@ -25,7 +25,7 @@ $(eval $(call gb_Library_add_defs,sm,\
 	-DSM_DLLIMPLEMENTATION \
 ))
 
-$(eval $(call gb_Library_use_external,sm,boost_headers))
+$(eval $(call gb_Library_use_external,sm,boost_headers,icu_headers))
 
 $(eval $(call gb_Library_use_custom_headers,sm,\
 	officecfg/registry \