Include <string.h> for the strcmp function.  This improves C99
compiler compatibility because it avoids an implicit function
declaration.

Submitted upstream:

  <https://lists.gnu.org/archive/html/libtool-patches/2023-01/msg00000.html>

diff --git a/tests/lt_dlopen_a.at b/tests/lt_dlopen_a.at
index 95f18a0df165d41d..d3948738e4199cc6 100644
--- a/tests/lt_dlopen_a.at
+++ b/tests/lt_dlopen_a.at
@@ -27,6 +27,7 @@ AT_KEYWORDS([libltdl lt_dlopen_a])
 AT_DATA([main.c],
 [[#include <ltdl.h>
 #include <stdio.h>
+#include <string.h>
 
 /* This dlopen() in the main executable should override any dlopen()s in
    shared libraries etc.