dtls: fix some more compiler warnings

dtls: fix some more compiler warnings

gcc-4.9.2:
gstdtlsagent.c:114:1: error: old-style function definition
gstdtlsconnection.c:253:3: error: ISO C90 forbids mixed declarations and code
gstdtlsconnection.c:291:3: error: ISO C90 forbids mixed declarations and code
gstdtlsconnection.c:391:3: error: ISO C90 forbids mixed declarations and code
gstdtlsconnection.c:434:3: error: ISO C90 forbids mixed declarations and code
gstdtlsconnection.c:773:1: error: 'BIO_s_gst_dtls_connection' was used with no prototype before its definition
gstdtlsconnection.c:773:1: error: old-style function definition