make ng-spice use the proper header directory

package/ng-spice-rework/ng-spice-rework.mk

@@ -11,6 +11,8 @@ NG_SPICE_REWORK_INSTALL_STAGING = NO
 NG_SPICE_REWORK_INSTALL_TARGET = YES
 NG_SPICE_REWORK_INSTALL_TARGET_OPT = DESTDIR=$(TARGET_DIR) install
 
+NG_SPICE_REWORK_CONF_OPT = CFLAGS="-I$(STAGING_DIR)/usr/include"
+
 NG-SPICE-REWORK_DEPENDENCIES = uclibc 
 
 $(eval $(call AUTOTARGETS,package,ng-spice-rework))