Use stdlib.h for malloc(), getenv() and realloc(). Fixes the following
build failure:

unixos.c:42:14: error: conflicting types for 'malloc'
 extern char *malloc();
              ^
Makefile:168: recipe for target 'unixos.o' failed
make[3]: *** [unixos.o] Error 1

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>