Fix cmake to add big endian compile option

Without this fix, mediastreamer2 doesn't have this compile option (CMakeLists.txt#L669) with CMake and it might create RTP packet with wrong payload type in header due to markbit field update (src/otherfilters/msrtp.c#L633)

CMakeLists.txt

@@ -100,9 +100,6 @@ check_symbol_exists(sendmsg "sys/socket.h" HAVE_SENDMSG)
 
 include(TestBigEndian)
 test_big_endian(WORDS_BIGENDIAN)
-if(WORDS_BIGENDIAN)
-	set(ORTP_BIGENDIAN 1)
-endif()
 
 
 include_directories(
@@ -116,6 +113,9 @@ endif()
 
 
 set(ORTP_CPPFLAGS ${BCTOOLBOX_CPPFLAGS})
+if(WORDS_BIGENDIAN)
+	list(APPEND ORTP_CPPFLAGS "-DORTP_BIGENDIAN")
+endif()
 if(ENABLE_STATIC)
 	list(APPEND ORTP_CPPFLAGS "-DORTP_STATIC")
 endif()