fsearch: Include <unistd.h> for the usleep function

This improves compatibility with compilers which do not support
implicit function declarations.

Upstream fsearch does not call usleep and is therefore not affected
by this issue.

src/plugins/filemanager/dfmplugin-search/3rdparty/fsearch/fsearch_thread_pool.c

@@ -20,6 +20,8 @@
 
 #include "fsearch_thread_pool.h"
 #include <sys/time.h>
+#include <unistd.h>
+
 struct _FsearchThreadPool {
     GList *threads;
     uint32_t num_threads;