package/alsa-lib: ucm: add BR2_USE_MMU dependency

Build fails with:
ucm_exec.c: In function 'uc_mgr_exec':
ucm_exec.c:229:13: error: implicit declaration of function 'fork' [-Wimplicit-function-declaration]
  229 |         p = fork();
      |             ^~~~

ucm uses fork() so add BR2_USE_MMU dependency for it.

Fixes: https://autobuild.buildroot.org/results/a287d3eef7ea5ea9a85b5b35d14e3698df4ecd01/build-end.log
Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>
(cherry picked from commit 928494b7859e1b2ab44d56a17dc8fcec8a661780)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/alsa-lib/Config.in

@@ -69,6 +69,7 @@ config BR2_PACKAGE_ALSA_LIB_SEQ
 config BR2_PACKAGE_ALSA_LIB_UCM
 	bool "ucm"
 	default y
+	depends on BR2_USE_MMU # fork()
 	select BR2_PACKAGE_ALSA_LIB_MIXER
 
 config BR2_PACKAGE_ALSA_LIB_ALISP