pw_boot_cortex_m: Fix function definition

Fixes the function signature of pw_boot_Entry() to match the
declaration.

Bug: b/239856986
Change-Id: Ia273b7fb117a34fa4dfe132d7bbf0c2608d97210
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/103789
Reviewed-by: Keir Mierle <keir@google.com>
Pigweed-Auto-Submit: Armando Montanez <amontanez@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pw_boot_cortex_m/core_init.c b/pw_boot_cortex_m/core_init.c
index cbb8af3..d2d6db3 100644
--- a/pw_boot_cortex_m/core_init.c
+++ b/pw_boot_cortex_m/core_init.c
@@ -107,7 +107,7 @@
 //
 // This function runs immediately at boot because it is at index 1 of the
 // interrupt vector table.
-void pw_boot_Entry() {
+void pw_boot_Entry(void) {
   // Disable interrupts.
   //
   // Until pw_boot_PreStaticMemoryInit() has completed, depending on the