Just declare gettid()

Author: grynspan

Sources/_TestingInternals/include/Stubs.h

@@ -152,6 +152,14 @@ static int swt_siginfo_t_si_status(const siginfo_t *siginfo) {
 #endif
 #endif
 
+#if defined(__linux__) || defined(__ANDROID__)
+/// Get the current thread's ID.
+///
+/// This function is redeclared here because it is guarded by `_GNU_SOURCE` in
+/// the platform headers.
+SWT_EXTERN pid_t gettid(void);
+#endif
+
 /// Get the value of `EEXIST`.
 ///
 /// This function is provided because `EEXIST` is a complex macro in wasi-libc