progname: Include <procinfo.h> if available

We need this header on AIX. Missed transplanting the code from the AIX
porting system.

Fixes: commit 9fa06763a1

configure.ac

@@ -179,6 +179,7 @@ AC_CHECK_HEADERS([\
   pwd.h \
   grp.h \
   stdio_ext.h \
+  procinfo.h \
 ])
 
 # Checks for typedefs, structures, and compiler characteristics.

src/progname.c

@@ -35,6 +35,9 @@
 #include <errno.h>
 #include <string.h>
 #include <stdlib.h>
+#ifdef HAVE_PROCINFO_H
+#include <procinfo.h>
+#endif
 #ifdef _WIN32
 #include <Windows.h>
 #include <shlwapi.h>