Fix CMake MSVC generation.

Move the add_dependencies() call for aom_pc to within
the if(MSVC) block, where it belongs.

BUG=aomedia:722

Change-Id: If25a6c83a981afc6d900e89e31db528af13b9e93

CMakeLists.txt

@@ -216,8 +216,8 @@ if (NOT MSVC)
       -P "${AOM_ROOT}/build/cmake/pkg_config.cmake"
       COMMENT "Writing aom.pc"
       VERBATIM)
+  add_dependencies(aom_pc aom_version)
 endif ()
-add_dependencies(aom_pc aom_version)
 
 # TODO(tomfinegan): Move rtcd target setup where it belongs for each rtcd
 # source.