os: add missing header for BSDs after fd4be8b (#16)

src/os/Process.cpp:105:9: error: use of undeclared identifier 'sigemptyset'
  105 |         sigemptyset(&set);
      |         ^
src/os/Process.cpp:106:21: error: use of undeclared identifier 'SIG_SETMASK'
  106 |         sigprocmask(SIG_SETMASK, &set, NULL);
      |                     ^

src/os/Process.cpp

@@ -1,6 +1,7 @@
 #include <hyprutils/os/Process.hpp>
 using namespace Hyprutils::OS;
 
+#include <csignal>
 #include <cstdio>
 #include <unistd.h>
 #include <cstring>