Fix a case of `unused-but-set-variable`

The `unused-but-set-variable` warning is raised on MacOS from the
`posix_fadvise` standin macro, since offset is often otherwise unused. Add a
cast to absorb this warning.
Signed-off-by: Trevor Gross <tmgross@umich.edu>

extra/mariabackup/common.h

@@ -143,7 +143,7 @@ static inline ATTRIBUTE_FORMAT(printf, 1,2) ATTRIBUTE_NORETURN void die(const ch
 # define POSIX_FADV_NORMAL
 # define POSIX_FADV_SEQUENTIAL
 # define POSIX_FADV_DONTNEED
-# define posix_fadvise(a,b,c,d) do {} while(0)
+# define posix_fadvise(fd, offset, len, advice) do { (void)offset; } while(0)
 #endif
 
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