libs.mk: don't include x86inc.asm in codec src list

this file is an include and doesn't need to be built on its own.
fixes:
ranlib: file: libvpx_g.a(x86inc.asm.o) has no symbols

Change-Id: I89504e37ff0a4488489af7b9b7e09fb32acc4853
diff --git a/libs.mk b/libs.mk
index 2b072b6..1e01639 100644
--- a/libs.mk
+++ b/libs.mk
@@ -170,7 +170,7 @@
 CODEC_SRCS-$(BUILD_LIBVPX) += $(BUILD_PFX)vpx_config.c
 INSTALL-SRCS-no += $(BUILD_PFX)vpx_config.c
 ifeq ($(ARCH_X86)$(ARCH_X86_64),yes)
-CODEC_SRCS-$(BUILD_LIBVPX) += third_party/x86inc/x86inc.asm
+INSTALL-SRCS-$(CONFIG_CODEC_SRCS) += third_party/x86inc/x86inc.asm
 endif
 CODEC_EXPORTS-$(BUILD_LIBVPX) += vpx/exports_com
 CODEC_EXPORTS-$(CONFIG_ENCODERS) += vpx/exports_enc