SMB: Address a clang warning

The warning was:
./print-smb.c:28:15: warning: no previous extern declaration for non-static
      variable 'startbuf' [-Wmissing-variable-declarations]
const u_char *startbuf = NULL;
              ^
diff --git a/print-smb.c b/print-smb.c
index f691491..b3f855b 100644
--- a/print-smb.c
+++ b/print-smb.c
@@ -25,6 +25,8 @@
 static int request = 0;
 static int unicodestr = 0;
 
+extern const u_char *startbuf;
+
 const u_char *startbuf = NULL;
 
 struct smbdescript {