loader: unconditionally include unistd.h and stdlib.h

loader: unconditionally include unistd.h and stdlib.h

Otherwise we would fail with "implicit declaration of function" geteuid
and getenv respectively.

To trigger (re)move the libdrm.pc file and use the following:

 $ ./autogen.sh --disable-egl --disable-gbm --disable-dri \
    --with-dri-drivers=swrast --with-gallium-drivers=swrast
 $ make

Cc: Vinson Lee <vlee@freedesktop.org>
Fixes: 3f462050c ("loader: Add an environment variable to override driver name choice.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99701
v2: [Emil: handle stdlib.h add commit message]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>