libbpf: Fix various errors when build libbpf in Android

In Android, we build C code using clang with [-Werror,-Wsign-compare]
flags. This causes various errors with libbpf.c and perf_reader.c such
as "error: comparison of integers of different signs: 'int' and
'unsigned long'". Let us fix them since it is a good idea to them
anyway.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>