qxl: switch to constants within BUILD_BUG_ON

We are switching BUILD_BUG_ON to verify that it's parameter is a
compile-time constant, and it turns out that some gcc versions
(specifically gcc (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609) are
not smart enough to figure it out for expressions involving local
variables. This is harmless but means that the check is ineffective for
these platforms.  To fix, replace variables with macros.
Reported-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

hw/display/qxl.c

@@ -306,12 +306,11 @@ void qxl_spice_reset_cursor(PCIQXLDevice *qxl)
 
 static ram_addr_t qxl_rom_size(void)
 {
-    uint32_t required_rom_size = sizeof(QXLRom) + sizeof(QXLModes) +
-                                 sizeof(qxl_modes);
-    uint32_t rom_size = 8192; /* two pages */
+#define QXL_REQUIRED_SZ (sizeof(QXLRom) + sizeof(QXLModes) + sizeof(qxl_modes))
+#define QXL_ROM_SZ 8192
 
-    QEMU_BUILD_BUG_ON(required_rom_size > rom_size);
-    return rom_size;
+    QEMU_BUILD_BUG_ON(QXL_REQUIRED_SZ > QXL_ROM_SZ);
+    return QXL_ROM_SZ;
 }
 
 static void init_qxl_rom(PCIQXLDevice *d)