Fix build with GLM 0.9.9.0, tdf#118070

One of the test files was forgotten in 953c4add

Change-Id: I0315433c688ff7edb2ba599bbef203cac9de3022
Reviewed-on: https://gerrit.libreoffice.org/55554Reviewed-by: Rene Engelhard <rene@debian.org>
Tested-by: Rene Engelhard <rene@debian.org>

vcl/Executable_icontest.mk

@@ -19,6 +19,12 @@ $(eval $(call gb_Executable_use_externals,icontest,\
 ))
 endif
 
+ifeq ($(SYSTEM_GLM),TRUE)
+$(eval $(call gb_Executable_add_defs,icontest,\
+    -DGLM_ENABLE_EXPERIMENTAL \
+))
+endif
+
 $(eval $(call gb_Executable_use_api,icontest,\
     offapi \
     udkapi \