tools/libs: Fix build following c/s 56dccee3f, take 2

The fix for c/s 01ba8f62b6 was speculative given no local repro.  It turns
out that it didn't fix the problem.

The $(AUTOINCS) variable needs to be visible before libs.mk is included, to
have any effect.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libs/toolcore/Makefile

@@ -4,6 +4,7 @@ include $(XEN_ROOT)/tools/Rules.mk
 MAJOR	= 1
 MINOR	= 0
 LIBNAME  := toolcore
+AUTOINCS := include/_xentoolcore_list.h
 
 SRCS-y	+= handlereg.c
 
@@ -11,8 +12,6 @@ include $(XEN_ROOT)/tools/libs/libs.mk
 
 $(PKG_CONFIG_LOCAL): PKG_CONFIG_INCDIR = $(XEN_LIBXENTOOLCORE)/include
 
-AUTOINCS=include/_xentoolcore_list.h
-
 $(LIB_OBJS): $(AUTOINCS)
 $(PIC_OBJS): $(AUTOINCS)