python: add include guard for python-host.mk and include in Makefile

So that it can be included twice.
python-package.mk include python-host.mk, but at some point
in time python-host.mk is not exported to staging_dir yet.

So, to have in python's Makefile, we need to include it
in the Makefile, and to prevent double inclusion (if
python-host.mk is exported from a previous build), the
include guard is added.

Also, moved the includes after some vars were set.

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

lang/python/Makefile

@@ -7,6 +7,9 @@
 
 include $(TOPDIR)/rules.mk
 
+# This file provides the necsessary host build variables
+include ./files/python-host.mk
+
 # The file included below defines PYTHON_VERSION
 include ./files/python-package.mk
 

lang/python/files/python-host.mk

@@ -5,6 +5,9 @@
 # See /LICENSE for more information.
 #
 
+ifneq ($(__python_host_mk_inc),1)
+__python_host_mk_inc=1
+
 # Compatibility fallback for older OpenWrt and LEDE versions
 ifeq ($(STAGING_DIR_HOSTPKG),)
   $(warning STAGING_DIR_HOSTPKG is unset - falling back to $$(STAGING_DIR)/host)
@@ -71,3 +74,4 @@ define Build/Compile/HostPyMod
 		$(3))
 endef
 
+endif # __python_host_mk_inc