vpxenc: Add vpxenc.h and move/rename the global_config struct

- Rename the struct to VpxEncoderConfig.
- The idea behind this is to enable checking the global settings against
  stream specific settings in source files other than vpxenc.c.

Change-Id: Ic736cbb714845b9466acb34671780d65b83ad1a8
diff --git a/examples.mk b/examples.mk
index 36d20df..2600a9d 100644
--- a/examples.mk
+++ b/examples.mk
@@ -37,7 +37,7 @@
 vpxdec.GUID                  = BA5FE66F-38DD-E034-F542-B1578C5FB950
 vpxdec.DESCRIPTION           = Full featured decoder
 UTILS-$(CONFIG_ENCODERS)    += vpxenc.c
-vpxenc.SRCS                 += args.c args.h y4minput.c y4minput.h
+vpxenc.SRCS                 += args.c args.h y4minput.c y4minput.h vpxenc.h
 vpxenc.SRCS                 += ivfdec.c ivfdec.h
 vpxenc.SRCS                 += ivfenc.c ivfenc.h
 vpxenc.SRCS                 += tools_common.c tools_common.h