Allow `core:sys/windows` to build on Windows only

core/sys/win32/crt.odin

@@ -1,3 +1,4 @@
+// +build windows
 package win32
 
 import "core:strings";

core/sys/windows/advapi32.odin

@@ -1,3 +1,4 @@
+// +build windows
 package sys_windows
 
 foreign import advapi32 "system:Advapi32.lib"

core/sys/windows/bcrypt.odin

@@ -1,3 +1,4 @@
+// +build windows
 package sys_windows
 
 foreign import bcrypt "system:Bcrypt.lib"

core/sys/windows/kernel32.odin

@@ -1,3 +1,4 @@
+// +build windows
 package sys_windows
 
 foreign import kernel32 "system:Kernel32.lib"

core/sys/windows/netapi32.odin

@@ -1,3 +1,4 @@
+// +build windows
 package sys_windows
 
 foreign import netapi32 "system:Netapi32.lib"

core/sys/windows/ntdll.odin

@@ -1,3 +1,4 @@
+// +build windows
 package sys_windows
 
 foreign import ntdll_lib "system:ntdll.lib"

core/sys/windows/shell32.odin

@@ -1,3 +1,4 @@
+// +build windows
 package sys_windows
 
 foreign import shell32 "system:Shell32.lib"

core/sys/windows/synchronization.odin

@@ -1,3 +1,4 @@
+// +build windows
 package sys_windows
 
 foreign import Synchronization "system:Synchronization.lib"

core/sys/windows/types.odin

@@ -1,3 +1,4 @@
+// +build windows
 package sys_windows
 
 import "core:c"

core/sys/windows/userenv.odin

@@ -1,3 +1,4 @@
+// +build windows
 package sys_windows
 
 foreign import userenv "system:Userenv.lib"

core/sys/windows/util.odin

@@ -1,3 +1,4 @@
+// +build windows
 package sys_windows
 
 import "core:strings"

core/sys/windows/ws2_32.odin

@@ -1,3 +1,4 @@
+// +build windows
 package sys_windows
 
 foreign import ws2_32 "system:Ws2_32.lib"