add public declaration for optreset under appropriate feature profiles

commit 030e52639248ac8417a4934298caa78c21a228d1 added optreset, a BSD
extension to getopt duplicating the functionality (also an extension)
of setting optind to 0, but failed to provide a public declaration for
it. according to the BSD documentation and headers, the application is
not supposed to need to provide its own declaration.

include/unistd.h

@@ -176,6 +176,7 @@ long syscall(long, ...);
 int execvpe(const char *, char *const [], char *const []);
 int issetugid(void);
 int getentropy(void *, size_t);
+extern int optreset;
 #endif
 
 #ifdef _GNU_SOURCE

src/misc/getopt.c

@@ -1,3 +1,4 @@
+#define _BSD_SOURCE
 #include <unistd.h>
 #include <wchar.h>
 #include <string.h>