without it /usr/include/bits/alltypes.h defines key_t as int

--- a/tools/cert_create/Makefile
+++ b/tools/cert_create/Makefile
@@ -43,7 +43,7 @@
 include ${PLAT_CERT_CREATE_HELPER_MK}
 endif
 
-HOSTCCFLAGS := -Wall -std=c99
+HOSTCCFLAGS := -Wall -std=c99 -D__DEFINED_key_t=1
 
 ifeq (${DEBUG},1)
   HOSTCCFLAGS += -g -O0 -DDEBUG -DLOG_LEVEL=40