fix(libc): memset inclusion to libc makefiles

Fix [1] is introducing another problem in that memset is added twice to
the libc makefile when OVERRIDE_LIBC=1 (the C and asm implementations).

Correct by adding memset.c when OVERRIDE_LIBC=0 and memset.S when
OVERRIDE_LIBC=1.

[1] https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/26091

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: Ie4b7e04880d4cd871e7b51cd8ff5bddcf8d0308c

lib/libc/libc.mk

@@ -6,3 +6,6 @@
 #
 
 include lib/libc/libc_common.mk
+
+LIBC_SRCS	+=	$(addprefix lib/libc/,		\
+			memset.c)

lib/libc/libc_common.mk

@@ -14,7 +14,6 @@ LIBC_SRCS	:=	$(addprefix lib/libc/,		\
 			memcpy_s.c			\
 			memmove.c			\
 			memrchr.c			\
-			memset.c			\
 			printf.c			\
 			putchar.c			\
 			puts.c				\