Add includes for syscall
Closes: https://bugs.gentoo.org/976333
From: https://github.com/shadps4-emu/shadPS4/commit/c68a8baa947fabfd74efaca1ba027ca15abf73db

--- a/src/core/tls.cpp
+++ b/src/core/tls.cpp
@@ -21,6 +21,8 @@
 #if defined(__linux__) && defined(ARCH_X86_64)
 #include <asm/prctl.h>
 #include <sys/prctl.h>
+#include <sys/syscall.h>
+#include <unistd.h>
 #endif
 
 namespace Core {