msgpack-c: don't check for GTest

Not available with OpenWrt. Fixes compilation when host GTest is
present.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
(cherry picked from commit 7f34b03356)

libs/msgpack-c/patches/010-no-gtest.patch

@@ -0,0 +1,10 @@
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -148,7 +148,6 @@ FOREACH (F ${PREPROCESSOR_FILES})
+    FILE (GENERATE OUTPUT ${CMAKE_CURRENT_SOURCE_DIR}/include/msgpack/${F} CONTENT ${CONTENT})
+ ENDFOREACH ()
+ 
+-FIND_PACKAGE (GTest)
+ FIND_PACKAGE (ZLIB)
+ FIND_PACKAGE (Threads)
+ IF (GTEST_FOUND AND ZLIB_FOUND AND THREADS_FOUND AND NOT "${MSGPACK_FUZZ_REGRESSION}" STREQUAL "ON")