nl_langinfo and CODESET are undefined in a musl system. Instead of
uncondionally including langinfo.h, this change includes include/nls.h
which has the tests and work-arounds for systems that don't have these
features. This is similar to how other programs within procps include
langinfo.h via nls.h
References:
procps-ng/procps!130
Signed-off-by: Craig Small <csmall@dropbear.xyz>