Include all lxcmntent.h function declarations on Bionic

Include all lxcmntent.h function declarations on Bionic

Newer versions of Android (5.0+, aka API Level 21+) include mntent.h,
which declares setmntent and endmntent. This hits an edge
case with the preprocessor checks in lxcmntent.h because HAVE_SETMNTENT
and HAVE_ENDMNTENT are both defined (in Bionic's mntent.h), but conf.c
always includes lxcmntent.h on Bionic! As a result, we get compiler
warnings of implicit function declarations for setmntent endmntent.

This patch always includes setmntent/endmntent/hasmntopt function
declarations on Bionic, which gets rid of these warnings.

Signed-off-by: Preetam D'Souza <preetamjdsouza@gmail.com>