cmake: Link pthreads into aom target.

Link pthread into the aom library target instead of
looping over all executable targets and making each
depend on pthread.

BUG=aomedia:2071

Change-Id: I93766e16e47fb74844180f20c7e43ba1e7a5b0ed

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1902da27..17ab72d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -513,9 +513,7 @@
 
 if(HAVE_PTHREAD_H AND CONFIG_MULTITHREAD)
   find_package(Threads)
-  foreach(app_target ${AOM_APP_TARGETS})
-    target_link_libraries(${app_target} ${AOM_LIB_LINK_TYPE} Threads::Threads)
-  endforeach()
+  target_link_libraries(aom ${AOM_LIB_LINK_TYPE} Threads::Threads)
 endif()
 
 if(XCODE)