Separate `__has_attribute` from `defined(__has_attribute)`

Fix Visual C warnings:
```
regenc.h(121): warning C4067: unexpected tokens following preprocessor directive - expected a newline
```

1 files changed, 3 insertions, 1 deletions

diff --git a/regenc.h b/regenc.h
index b353ae0f9e..4b4d21a715 100644
--- a/regenc.h
+++ b/regenc.h
@@ -118,8 +118,10 @@ typedef struct {
 
 typedef struct {
   short int len;
-#if defined(__has_attribute) && __has_attribute(nonstring)
+#if defined(__has_attribute)
+# if __has_attribute(nonstring)
   __attribute__((nonstring))
+# endif
 #endif
   const UChar name[6];
   int       ctype;