cmake: fix VMAF_CFLAGS handling

This may be a list of items, e.g., -pthread;-I...
Fixes missing libvmaf.h include errors due to the include path not being
added to the compile flags.

Bug: aomedia:3464
Change-Id: I5c445da148018f4ebe97663d7fe9fc7bbd5448be
(cherry picked from commit eff5e508b3be88b54e85e2020efb8e488c509186)

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5b18659..371ecd6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -648,7 +648,9 @@
       target_include_directories(aom PRIVATE ${VMAF_INCLUDE_DIRS})
       target_include_directories(aom_dsp_encoder PRIVATE ${VMAF_INCLUDE_DIRS})
       if(VMAF_CFLAGS)
-        append_compiler_flag("${VMAF_CFLAGS}")
+        foreach(flag "${VMAF_CFLAGS}")
+          append_compiler_flag("${flag}")
+        endforeach()
       endif()
     else()
       message(FATAL_ERROR "CONFIG_TUNE_VMAF error: pkg-config not found.")