curl_rtmp: fix missing-variable-declarations warnings

clang complains:

curl_rtmp.c:61:27: error: no previous extern declaration for non-static variable 'Curl_handler_rtmp' [-Werror,-Wmissing-variable-declarations]
curl_rtmp.c:81:27: error: no previous extern declaration for non-static variable 'Curl_handler_rtmpt' [-Werror,-Wmissing-variable-declarations]
curl_rtmp.c:101:27: error: no previous extern declaration for non-static variable 'Curl_handler_rtmpe' [-Werror,-Wmissing-variable-declarations]
curl_rtmp.c:121:27: error: no previous extern declaration for non-static variable 'Curl_handler_rtmpte' [-Werror,-Wmissing-variable-declarations]
curl_rtmp.c:141:27: error: no previous extern declaration for non-static variable 'Curl_handler_rtmps' [-Werror,-Wmissing-variable-declarations]
curl_rtmp.c:161:27: error: no previous extern declaration for non-static variable 'Curl_handler_rtmpts' [-Werror,-Wmissing-variable-declarations]

Fix this by including the header file.