format: Add missing braces around 'if' clause

When compiling bmon with gcc 6.1 it complains with the following
warning:

out_format.c: In function ‘get_token’:
out_format.c:134:10: warning: this ‘if’ clause does not guard... [-Wmisleading-indentation]
   } else if (!strncasecmp(token+5, "txrate:", 7))
          ^~
out_format.c:136:4: note: ...this statement, but the latter is misleadingly indented as if it is guarded by the ‘if’
    return buf;
    ^~~~~~

Indeed, the 'return buf' should only be executed if it was snprintf()'ed
to. Otherwise "unknown" should be returned. Fix this by adding braces.
Also use the 'type' variable in strncasecmp() as in the other checks.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

src/out_format.c

@@ -131,9 +131,10 @@ static char *get_token(struct element_group *g, struct element *e,
 		} else if (!strncasecmp(type, "rxrate:", 7)) {
 			snprintf(buf, len, "%.2f", a->a_rx_rate.r_rate);
 			return buf;
-		} else if (!strncasecmp(token+5, "txrate:", 7))
+		} else if (!strncasecmp(type, "txrate:", 7)) {
 			snprintf(buf, len, "%.2f", a->a_tx_rate.r_rate);
 			return buf;
+		}
 	}
 
 	fprintf(stderr, "Unknown field \"%s\"\n", token);