package/psmisc: fix musl build

package/psmisc: fix musl build

Add upstream commit fixing a build error not yet been found by the
autobuilders:

pstree.c: In function ‘get_threadname’:
pstree.c:798:15: error: ‘PATH_MAX’ undeclared (first use in this function)
     char path[PATH_MAX + 1];
               ^
pstree.c:798:15: note: each undeclared identifier is reported only once for each function it appears in
pstree.c:798:10: warning: unused variable ‘path’ [-Wunused-variable]
     char path[PATH_MAX + 1];
          ^

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>