Fix GH-4449: adding strftime* for illumos.

[ref](https://smartos.org/man/3C/strftime)
[ref](https://docs.oracle.com/cd/E88353_01/html/E37843/strftime-3c.html)

Author: devnexen

libc-test/semver/solarish.txt

@@ -78,3 +78,5 @@ posix_spawnattr_setsigmask
 posix_spawnp
 recvmsg
 sendmsg
+strftime
+strftime_l

src/unix/solarish/mod.rs

@@ -3202,6 +3202,20 @@ extern "C" {
     pub fn arc4random_uniform(upper_bound: u32) -> u32;
 
     pub fn secure_getenv(name: *const c_char) -> *mut c_char;
+
+    pub fn strftime(
+        s: *mut c_char,
+        maxsize: size_t,
+        format: *const c_char,
+        timeptr: *const crate::tm,
+    ) -> size_t;
+    pub fn strftime_l(
+        s: *mut c_char,
+        maxsize: size_t,
+        format: *const c_char,
+        timeptr: *const crate::tm,
+        loc: crate::locale_t,
+    ) -> size_t;
 }
 
 #[link(name = "sendfile")]