Fix truncation warnings in posix/tst-glob_symlinks.c.

The test posix/tst-glob_symlinks.c fails to build with GCC mainline:

tst-glob_symlinks.c: In function 'do_test':
tst-glob_symlinks.c:124:30: error: 'snprintf' output may be truncated before the last format character [-Werror=format-truncation=]
   snprintf (buf, sizeof buf, "%s?", dangling_link);
                              ^~~~~
tst-glob_symlinks.c:124:3: note: 'snprintf' output between 2 and 4097 bytes into a destination of size 4096
   snprintf (buf, sizeof buf, "%s?", dangling_link);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tst-glob_symlinks.c:128:30: error: 'snprintf' output may be truncated before the last format character [-Werror=format-truncation=]
   snprintf (buf, sizeof buf, "%s*", dangling_link);
                              ^~~~~
tst-glob_symlinks.c:128:3: note: 'snprintf' output between 2 and 4097 bytes into a destination of size 4096
   snprintf (buf, sizeof buf, "%s*", dangling_link);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This patch fixes the test to avoid such truncation warnings by
increasing the buffer in question by one byte, to ensure it can hold
any possible result of %s? or %s* formats where %s comes from a buffer
of size PATH_MAX.

Tested compilation with build-many-glibcs.py for aarch64-linux-gnu.

	* posix/tst-glob_symlinks.c (do_test): Increase size of buf.