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 <25a259f709b0f1b620605d2518271a846d8826e5@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 <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler