Auto merge of #3253 - devnexen:dflybsd_malloc_usable_size, r=JohnTitor

dragonflybsd supports malloc_usable_size too

Author: bors

libc-test/semver/dragonfly.txt

@@ -1383,6 +1383,7 @@ lutimes
 lwp_rtprio
 lwpid_t
 madvise
+malloc_usable_size
 mcontext_t
 memmem
 memrchr

src/unix/bsd/freebsdlike/freebsd/mod.rs

@@ -5273,7 +5273,6 @@ extern "C" {
     pub fn fls(value: ::c_int) -> ::c_int;
     pub fn flsl(value: ::c_long) -> ::c_int;
     pub fn flsll(value: ::c_longlong) -> ::c_int;
-    pub fn malloc_usable_size(ptr: *const ::c_void) -> ::size_t;
     pub fn malloc_stats_print(
         write_cb: unsafe extern "C" fn(*mut ::c_void, *const ::c_char),
         cbopaque: *mut ::c_void,

src/unix/bsd/freebsdlike/mod.rs

@@ -1571,6 +1571,7 @@ extern "C" {
         mode: ::mode_t,
         dev: dev_t,
     ) -> ::c_int;
+    pub fn malloc_usable_size(ptr: *const ::c_void) -> ::size_t;
     pub fn mincore(addr: *const ::c_void, len: ::size_t, vec: *mut ::c_char) -> ::c_int;
     pub fn newlocale(mask: ::c_int, locale: *const ::c_char, base: ::locale_t) -> ::locale_t;
     pub fn nl_langinfo_l(item: ::nl_item, locale: ::locale_t) -> *mut ::c_char;