python3: add PYTHON3_INC_DIR var to CPPFLAGS

The target's PYTHON3_INC_DIR should take precedence over the host's
include dir when cross-compiling.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

lang/python3/files/python3-package.mk

@@ -75,7 +75,7 @@ define Build/Compile/Py3Mod
 		CC="$(TARGET_CC)" \
 		CCSHARED="$(TARGET_CC) $(FPIC)" \
 		CFLAGS="$(TARGET_CFLAGS)" \
-		CPPFLAGS="$(TARGET_CPPFLAGS)" \
+		CPPFLAGS="$(TARGET_CPPFLAGS) -I$(PYTHON3_INC_DIR)" \
 		LDFLAGS="$(TARGET_LDFLAGS)" \
 		$(3) \
 		, \