core: stm32_etzpc: fix remaining build warning

Correct build warnings reported from the GCC traces below.
  core/drivers/stm32_etzpc.c:106:13: warning: ‘valid_decprot_id’ defined but not used [-Wunused-function]
   static bool valid_decprot_id(unsigned int id)

Fixes commit 54ca183 ("core: stm32_etzpc: fix build warnings") that
missed a warning occurrence.

Change-Id: Iacfd527dabf25ea02fd9a3c5e423441594aa31ec
Signed-off-by: Etienne Carriere <[email protected]>
Reviewed-on: https://gerrit.st.com/126679
Reviewed-by: CITOOLS <[email protected]>
Reviewed-by: Lionel DEBIEVE <[email protected]>
Tested-by: Lionel DEBIEVE <[email protected]>

core/drivers/stm32_etzpc.c

@@ -103,7 +103,7 @@ static uintptr_t etzpc_base(void)
 	return etzpc_dev.base;
 }
 
-static bool valid_decprot_id(unsigned int id)
+static bool __maybe_unused valid_decprot_id(unsigned int id)
 {
 	return id < (unsigned int)etzpc_dev.num_per_sec;
 }