fix(versal): add external declaration

This corrects the MISRA violation C2012-8.4:
A compatible declaration shall be visible when an object or
function with external linkage is defined.

Change-Id: I91817596c5de84b259a5dffcc01a7b1106a5b7a4
Signed-off-by: Maheedhar Bollapalli <maheedharsai.bollapalli@amd.com>
diff --git a/plat/xilinx/versal/plat_psci.c b/plat/xilinx/versal/plat_psci.c
index 7571e9d..9acf24e 100644
--- a/plat/xilinx/versal/plat_psci.c
+++ b/plat/xilinx/versal/plat_psci.c
@@ -126,7 +126,7 @@
 	plat_versal_gic_cpuif_enable();
 }
 
-void versal_pwr_domain_on_finish(const psci_power_state_t *target_state)
+static void versal_pwr_domain_on_finish(const psci_power_state_t *target_state)
 {
 	/* Enable the gic cpu interface */
 	plat_versal_gic_pcpu_init();