hw/pc_sysfw: Fix memory leak

Valgrind reported this memory leak which occured a few times.

Test scenario:

qemu-system-i386 (no arguments), only BIOS started, terminate with
monitor command (quit).

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Andreas Färber <afaerber@suse.de>

hw/pc_sysfw.c

@@ -85,6 +85,9 @@ static void pc_fw_add_pflash_drv(void)
     filename = qemu_find_file(QEMU_FILE_TYPE_BIOS, bios_name);
 
     opts = drive_add(IF_PFLASH, -1, filename, "readonly=on");
+
+    g_free(filename);
+
     if (opts == NULL) {
       return;
     }