ucm_exec.c: Include limits.h explicitly to fix build on musl

ucm_exec.c: Include limits.h explicitly to fix build on musl

Fixes:
| ../../../alsa-lib-1.2.5/src/ucm/ucm_exec.c: In function 'find_exec':
| ../../../alsa-lib-1.2.5/src/ucm/ucm_exec.c:43:18: error: 'PATH_MAX' undeclared (first use in this function)
|    43 |         char bin[PATH_MAX];
|       |                  ^~~~~~~~
| ../../../alsa-lib-1.2.5/src/ucm/ucm_exec.c:43:18: note: each undeclared identifier is reported only once for each function it appears in
| ../../../alsa-lib-1.2.5/src/ucm/ucm_exec.c: In function 'uc_mgr_exec':
| ../../../alsa-lib-1.2.5/src/ucm/ucm_exec.c:177:18: error: 'PATH_MAX' undeclared (first use in this function)
|   177 |         char bin[PATH_MAX];
|       |                  ^~~~~~~~

Fixes: https://github.com/alsa-project/alsa-lib/pull/145
Signed-off-by: Andreas Müller <schnitzeltony@gmail.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>