pci: provide prototype for pci_skip_dev outside of #if defined(CONFIG_DM_PCI_COMPAT)

The weak definition of pci_skip_dev from drivers/pci/pci_common.c is not
under CONFIG_DM_PCI_COMPAT, and that definition needs a previous
function prototype declaration to avoid W=1 build warnings.

That prototype is not available due to it being under CONFIG_DM_PCI_COMPAT,
so move it outside of that preprocessor block.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>