env: add missing <compiler.h> header file

Since commit af95f20 ("env: Create a new file for environment functions"),
a new header file exists.

So, this commit add a missing header file.

Fixes:

include/env.h:158:1: error: unknown type name ‘ulong’; did you mean ‘long’?
 ulong env_get_ulong(const char *name, int base, ulong default_val);
 ^~~~~
 long
include/env.h:158:49: error: unknown type name ‘ulong’; did you mean ‘long’?
 ulong env_get_ulong(const char *name, int base, ulong default_val);

Signed-off-by: Pierre-Jean Texier <pjtexier@koncepto.io>
Tested-by: Joris Offouga <offougajoris@gmail.com>
Tested-by: Heiko Schocher <hs@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>