blktap2: fix libgcrypt detection

If we want to check the functionality of libgcrypt, we shouldn't test
a function only exported by openssl, but instead the one actually used
in the code.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>

tools/blktap2/drivers/check_gcrypt

@@ -4,8 +4,7 @@ cat > .gcrypt.c << EOF
 #include <gcrypt.h>
 int main(void) 
 {
-    char dummy[14];
-    MD5("DUMMY", 5, dummy);
+    gcry_md_hash_buffer(GCRY_MD_MD5, NULL, NULL, 0);
     return 0; 
 }
 EOF