[cmake, deps] conjure common/httplib.h and remove global def for httplib macros (#3800)

httplib stuff done by @crueter on #3797

+ some extra stuff since the warning push/pop should be in header i fear :)

Signed-off-by: lizzie <lizzie@eden-emu.dev>

Co-authored-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3800
Reviewed-by: crueter <crueter@eden-emu.dev>
Co-authored-by: lizzie <lizzie@eden-emu.dev>
Co-committed-by: lizzie <lizzie@eden-emu.dev>

src/web_service/web_backend.cpp

@@ -9,17 +9,7 @@
 #include <string>
 
 #include <fmt/ranges.h>
-
-#ifdef __GNUC__
-#pragma GCC diagnostic push
-#ifndef __clang__
-#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
-#endif
-#endif
-#include <httplib.h>
-#ifdef __GNUC__
-#pragma GCC diagnostic pop
-#endif
+#include "common/httplib.h"
 
 #ifdef YUZU_BUNDLED_OPENSSL
 #include <openssl/cert.h>