install asm_offsets.h

Ensure vpx_ports/asm_offsets.h is installed with make dist

Change-Id: If9f32273fff975d60de1583b039dbbce8a7ccd27
diff --git a/libs.mk b/libs.mk
index 47709ba..3f600ce 100644
--- a/libs.mk
+++ b/libs.mk
@@ -89,6 +89,7 @@
 
 CODEC_SRCS-$(BUILD_LIBVPX) += build/make/version.sh
 CODEC_SRCS-$(BUILD_LIBVPX) += vpx/vpx_integer.h
+CODEC_SRCS-$(BUILD_LIBVPX) += vpx_ports/asm_offsets.h
 CODEC_SRCS-$(BUILD_LIBVPX) += vpx_ports/vpx_timer.h
 CODEC_SRCS-$(BUILD_LIBVPX) += vpx_ports/mem.h
 CODEC_SRCS-$(BUILD_LIBVPX) += $(BUILD_PFX)vpx_config.c