build/glsl: fix android build v2

Commit 77a3efc6b907943903190b385fdf107c4acfcdca broke android build that
sets its own value for GLSL_SRCDIR before including Makefile.sources.
Patch moves overriding the value after include, this works as GLSL_SRCDIR
variable gets expanded only later.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
diff --git a/src/glsl/Android.mk b/src/glsl/Android.mk
index 66c8bec..87a02f5 100644
--- a/src/glsl/Android.mk
+++ b/src/glsl/Android.mk
@@ -25,9 +25,9 @@
 
 LOCAL_PATH := $(call my-dir)
 
-GLSL_SRCDIR = .
 include $(LOCAL_PATH)/Makefile.sources
 
+GLSL_SRCDIR = .
 # ---------------------------------------
 # Build libmesa_glsl
 # ---------------------------------------