Fix gb_Library_use_externals call

...from d8f46f85 "enable ICU_HEADER includes for
starmath"

Change-Id: Idcafaeb6a43c4aa423a2c7a7ea4669d4fdaf3744

starmath/Library_sm.mk

@@ -25,7 +25,10 @@ $(eval $(call gb_Library_add_defs,sm,\
 	-DSM_DLLIMPLEMENTATION \
 ))
 
-$(eval $(call gb_Library_use_external,sm,boost_headers,icu_headers))
+$(eval $(call gb_Library_use_externals,sm, \
+    boost_headers \
+    icu_headers \
+))
 
 $(eval $(call gb_Library_use_custom_headers,sm,\
 	officecfg/registry \