contrib: update libvpx 48a3df8

- critical for building with iPhoneOS 12.0 SDK

Change-Id: I89f012d83d0403fb15b9071a28d1bb60a5ffb681

contrib/src/vpx/SHA512SUMS

-13b7eb515d026b302b8c484d6f06df067feb5aed4ceac953d85fe84fd684dab07e89ddbf80b4f395c6d9127709422156287e5754f49246050b2e18930adb3970  libvpx-v1.6.1.tar.gz
-8b3b766b550f8d86907628d7ed88035f9a2612aac21542e0fd5ad35b905eb82cbe1be02a1a24afce7a3bcc4766f62611971f72724761996b392136c40a1e7ff0  libvpx-v1.7.0.tar.gz
-3ad6ce36a9a3b044029c069147d69a24f48d9ef872123a587e728a9e20884b7d75cb052b1aed4a020f06314daa80ea1f78d30b829bf25653367c37ce4fe579fb  libvpx-8d56220599bac036f544fb3e35b6be6f71eeb765.tar.gz
\ No newline at end of file
+1bfa44bfb8069ccc672089a5b407d4326397c73f11bbb1b616f0c50cf1fe0087b96b545511f5280ded585ae72cc7f3f91dc0383c4d51f0400e37c1725b0c9de8  libvpx-48a3df87c0cced325f1bf95c72c8ac0f8392f919.tar.gz

contrib/src/vpx/rules.mak

 # libvpx
 
-VPX_HASH := v1.7.0
+VPX_HASH := 48a3df87c0cced325f1bf95c72c8ac0f8392f919
 VPX_URL := https://github.com/webmproject/libvpx/archive/$(VPX_HASH).tar.gz
 
 $(TARBALLS)/libvpx-$(VPX_HASH).tar.gz: