windows: provide DllMain stub for non-MSVC ABI

Part of preparation for upstreaming a Win32 application runtime
(see discussion #2563). This is one of three small build-related
fixes that unblock the Windows GNU-ABI library build.

When targeting Windows with GNU ABI, the existing `DllMain` declaration
falls through to `void` (a type), which Zig stdlib's `start.zig` then
attempts to call as a function via `root.DllMain(...)` - producing the
compile error "type 'type' not a function".

Restructure the conditional so that:
  - non-Windows builds keep `DllMain = void`
  - Windows + MSVC keeps the existing CRT-init handler (unchanged)
  - Windows + non-MSVC gets a no-op `BOOL` handler

This unblocks `zig build -Dtarget=native-native-gnu -Dapp-runtime=none`
on Windows.

src/main_c.zig

@@ -168,8 +168,7 @@ pub export fn ghostty_string_free(str: String) void {
 // but not MSVC. No upstream issue tracks this exact gap as of 2026-03-26.
 // Closest: Codeberg ziglang/zig #30936 (reimplement crt0 code).
 // Remove this DllMain when Zig handles MSVC DLL CRT init natively.
-pub const DllMain = if (builtin.os.tag == .windows and
-    builtin.abi == .msvc) struct {
+pub const DllMain = if (builtin.os.tag != .windows) void else if (builtin.abi == .msvc) struct {
     const BOOL = std.os.windows.BOOL;
     const HINSTANCE = std.os.windows.HINSTANCE;
     const DWORD = std.os.windows.DWORD;
@@ -200,7 +199,19 @@ pub const DllMain = if (builtin.os.tag == .windows and
             else => return TRUE,
         }
     }
-}.handler else void;
+}.handler else struct {
+    // GNU ABI: provide a no-op DllMain so Zig's start.zig doesn't
+    // try to call a type instead of a function.
+    const BOOL = std.os.windows.BOOL;
+    const HINSTANCE = std.os.windows.HINSTANCE;
+    const DWORD = std.os.windows.DWORD;
+    const LPVOID = std.os.windows.LPVOID;
+    const TRUE = std.os.windows.TRUE;
+
+    pub fn handler(_: HINSTANCE, _: DWORD, _: LPVOID) callconv(.winapi) BOOL {
+        return TRUE;
+    }
+}.handler;
 
 test "ghostty_string_s empty string" {
     const testing = std.testing;