x11vulkan: Use the correct SONAME of libX11-xcb.so.1 on Linux, etc.

On most Unix platforms supported by SDL, the canonical name used to load
a library at runtime includes its ABI major version, and the name
without a version is not guaranteed to exist on non-developer systems.
libX11-xcb.so.1 is correct on Linux, and probably on other Unix
platforms like FreeBSD.

A notable exception is OpenBSD, which apparently does not use
ABI-suffixed names, so continue to use libX11-xcb.so there.

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 7713a7eec7d953be16c3f5beccb5a9f3bb6c24d2)

src/video/x11/SDL_x11vulkan.c

@@ -32,8 +32,10 @@
 
 #if defined(__OpenBSD__)
 #define DEFAULT_VULKAN "libvulkan.so"
+#define DEFAULT_X11_XCB "libX11-xcb.so"
 #else
 #define DEFAULT_VULKAN "libvulkan.so.1"
+#define DEFAULT_X11_XCB "libX11-xcb.so.1"
 #endif
 
 /*
@@ -108,7 +110,7 @@ int X11_Vulkan_LoadLibrary(_THIS, const char *path)
     } else {
         const char *libX11XCBLibraryName = SDL_getenv("SDL_X11_XCB_LIBRARY");
         if (!libX11XCBLibraryName) {
-            libX11XCBLibraryName = "libX11-xcb.so";
+            libX11XCBLibraryName = DEFAULT_X11_XCB;
         }
         videoData->vulkan_xlib_xcb_library = SDL_LoadObject(libX11XCBLibraryName);
         if (!videoData->vulkan_xlib_xcb_library) {