build: Skip compiling rtmpdh.h if ffrtmpcrypt protocol is not enabled

The ffrtmpcrypt protocol depends on external libraries, which are
also required to compile the header file.

libavformat/Makefile

@@ -365,6 +365,7 @@ OBJS-$(CONFIG_TCP_PROTOCOL)              += tcp.o
 OBJS-$(CONFIG_TLS_PROTOCOL)              += tls.o
 OBJS-$(CONFIG_UDP_PROTOCOL)              += udp.o
 
+SKIPHEADERS-$(CONFIG_FFRTMPCRYPT_PROTOCOL) += rtmpdh.h
 SKIPHEADERS-$(CONFIG_NETWORK)            += network.h rtsp.h
 
 EXAMPLES  = metadata                                                    \