LibPthread: Make some variables static

Author: https://github.com/gunnarbeutner
Commit: https://github.com/SerenityOS/serenity/commit/f63f471b878
Pull-request: https://github.com/SerenityOS/serenity/pull/7518
Issue: https://github.com/SerenityOS/serenity/issues/7267
Reviewed-by: https://github.com/bgianfo

Userland/Libraries/LibPthread/pthread.cpp

@@ -29,9 +29,9 @@ using PthreadAttrImpl = Syscall::SC_create_thread_params;
 
 } // end anonymous namespace
 
-constexpr size_t required_stack_alignment = 4 * MiB;
-constexpr size_t highest_reasonable_guard_size = 32 * PAGE_SIZE;
-constexpr size_t highest_reasonable_stack_size = 8 * MiB; // That's the default in Ubuntu?
+static constexpr size_t required_stack_alignment = 4 * MiB;
+static constexpr size_t highest_reasonable_guard_size = 32 * PAGE_SIZE;
+static constexpr size_t highest_reasonable_stack_size = 8 * MiB; // That's the default in Ubuntu?
 
 #define __RETURN_PTHREAD_ERROR(rc) \
     return ((rc) < 0 ? -(rc) : 0)