Revert "package/Makefile.in should grab HOST_DIR headers using -isystem instead of -I."

This reverts commit 6f8162cf8c1abef7e0a4771fe0d6b26a28f5c2b6. This is
causing too many problems that are not easy to solve.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/Makefile.in

@@ -222,7 +222,7 @@ UNZIP := $(shell which unzip || type -p unzip) -q
 
 APPLY_PATCHES = support/scripts/apply-patches.sh $(if $(QUIET),-s)
 
-HOST_CPPFLAGS  = -isystem $(HOST_DIR)/usr/include
+HOST_CPPFLAGS  = -I$(HOST_DIR)/usr/include
 HOST_CFLAGS   ?= -O2
 HOST_CFLAGS   += $(HOST_CPPFLAGS)
 HOST_CXXFLAGS += $(HOST_CFLAGS)