android: build android sw winsys

This builds the static library libmesa_winsys_sw_android from winsys/sw.
diff --git a/src/gallium/Android.mk b/src/gallium/Android.mk
index 3a813bb..d2697c5 100644
--- a/src/gallium/Android.mk
+++ b/src/gallium/Android.mk
@@ -31,5 +31,8 @@
 	state_trackers/egl \
 	auxiliary
 
+# swrast
+SUBDIRS += winsys/sw/android
+
 mkfiles := $(patsubst %,$(GALLIUM_TOP)/%/Android.mk,$(SUBDIRS))
 include $(mkfiles)