Highway: Build header only

Change-Id: Ie52831eff9269291c8ee91a11a94895a8ecb8be6

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index bd96b60..a384ff2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -303,6 +303,7 @@
               "${AOM_ROOT}/third_party/highway/hwy/contrib/dot/dot-inl.h"
               "${AOM_ROOT}/third_party/highway/hwy/contrib/image/image.h"
               "${AOM_ROOT}/third_party/highway/hwy/contrib/math/math-inl.h")
+  add_definitions(-DHWY_HEADER_ONLY)
   add_library(aom_hwy OBJECT ${AOM_HIGHWAY_SOURCES})
   set(AOM_LIB_TARGETS ${AOM_LIB_TARGETS} aom_hwy)
 endif()