src/global.c: Fix conditional AES-CTR support

Most of libssh2 already has conditional support for AES-CTR according to
the LIBSSH2_AES_CTR crypto backend #define, but global.c needed fixing.

src/global.c

@@ -46,7 +46,9 @@ libssh2_init(int flags)
 {
     if (_libssh2_initialized == 0 && !(flags & LIBSSH2_INIT_NO_CRYPTO)) {
         libssh2_crypto_init();
+#if LIBSSH2_AES_CTR
         _libssh2_init_aes_ctr();
+#endif
     }
 
     _libssh2_initialized++;