Add wrapper for __msan_unpoinson() to reduce #ifdeffery

This isn't really necessary for the subsequent commit, but I expect that we'll
need to unpoison more often once we turn on msan in CI, so I think think this
change makes sense in the long run.

src/basic/alloc-util.h

@@ -8,6 +8,10 @@
 
 #include "macro.h"
 
+#if HAS_FEATURE_MEMORY_SANITIZER
+#  include <sanitizer/msan_interface.h>
+#endif
+
 typedef void (*free_func_t)(void *p);
 
 /* If for some reason more than 4M are allocated on the stack, let's abort immediately. It's better than
@@ -160,3 +164,9 @@ void* greedy_realloc0(void **p, size_t *allocated, size_t need, size_t size);
                 (ptr) = NULL;                   \
                 _ptr_;                          \
         })
+
+#if HAS_FEATURE_MEMORY_SANITIZER
+#  define msan_unpoison(r, s) __msan_unpoison(r, s)
+#else
+#  define msan_unpoison(r, s)
+#endif