Make eltt2 c99 compliant

Some implicit headerfiles were missing, if used without -std=gnu99.
Since we do not rely on gnu99 behavior by including the header files,
we can use -std=c99, which we state explicitly in the Makefile now.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>

Makefile

@@ -2,7 +2,7 @@
 # Copyright (c) Infineon Technologies AG
 
 CC=gcc
-CFLAGS=-Wall -Wextra -g
+CFLAGS=-Wall -Wextra -std=c99 -g
 
 all: eltt2