Include windows.h for atomics always in capi.c

src/core/capi.c

@@ -31,11 +31,13 @@
 #ifndef JANET_SINGLE_THREADED
 #ifndef JANET_WINDOWS
 #include <pthread.h>
-#else
-#include <windows.h>
 #endif
 #endif
 
+#ifdef JANET_WINDOWS
+#include <windows.h>
+#endif
+
 #ifdef JANET_USE_STDATOMIC
 #include <stdatomic.h>
 /* We don't need stdatomic on most compilers since we use compiler builtins for atomic operations.