blktap2: Check gcrypt library has MD5() function.

From: Dulloor <dulloor@gmail.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

tools/blktap2/drivers/check_gcrypt

@@ -2,7 +2,12 @@
 
 cat > .gcrypt.c << EOF
 #include <gcrypt.h>
-int main(void) { return 0; }
+int main(void) 
+{
+    char dummy[14];
+    MD5("DUMMY", 5, dummy);
+    return 0; 
+}
 EOF
 
 if $1 -o .gcrypt .gcrypt.c -lgcrypt 2>/dev/null ; then