tools/file2include: don't use malloc.h for malloc

stdlib.h is the header for malloc since at least c89/c90.
Previously this would fail to build on OpenBSD and fallback to the wrong
header:

In file included from u-boot/tools/file2include.c:21:
u-boot/include/malloc.h:875:5: error: function-like macro
      'CONFIG_IS_ENABLED' is not defined

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>