pem: fix -Wunused-but-set-variable

The loop never uses the value of 'line'.

Fixes this error with Clang 15:
```
crypto/pem/pem_lib.c:821:14: error: variable 'line' set but not used [-Werror,-Wunused-but-set-variable]
    int len, line, ret = 0, end = 0, prev_partial_line_read = 0, partial_line_read = 0;
             ^
1 error generated.
```

Signed-off-by: Sam James <sam@gentoo.org>

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #19450)

crypto/pem/pem_lib.c

@@ -818,7 +818,7 @@ static int get_header_and_data(BIO *bp, BIO **header, BIO **data, char *name,
 {
     BIO *tmp = *header;
     char *linebuf, *p;
-    int len, line, ret = 0, end = 0, prev_partial_line_read = 0, partial_line_read = 0;
+    int len, ret = 0, end = 0, prev_partial_line_read = 0, partial_line_read = 0;
     /* 0 if not seen (yet), 1 if reading header, 2 if finished header */
     enum header_status got_header = MAYBE_HEADER;
     unsigned int flags_mask;
@@ -830,7 +830,7 @@ static int get_header_and_data(BIO *bp, BIO **header, BIO **data, char *name,
     if (linebuf == NULL)
         return 0;
 
-    for (line = 0; ; line++) {
+    while(1) {
         flags_mask = ~0u;
         len = BIO_gets(bp, linebuf, LINESIZE);
         if (len <= 0) {