package/libtpms: fix build with host gcc >= 15.x, glibc >= 2.43

Fixes:
https://autobuild.buildroot.net/results/7a6/7a6ea7c513e8f1dcc32d868108bfb2a907d6fe85/

The previous change to src/tpm_library.c dates back to 2024:
https://github.com/stefanberger/libtpms/commits/master/src/tpm_library.c
so a backport to buildroot LTS branches should be considered.

Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
[Julien: reword title to mention glibc 2.43]
Signed-off-by: Julien Olivain <ju.o@free.fr>
(cherry picked from commit f773fd6787)
Signed-off-by: Thomas Perale <thomas.perale@mind.be>

package/libtpms/0001-Fix-a-compilation-error-in-TPMLIB_GetPlaintext.patch

@@ -0,0 +1,40 @@
+From fc8820cfaa8b5e17328f731df93911f6ab92443b Mon Sep 17 00:00:00 2001
+From: Stefan Berger <stefanb@linux.ibm.com>
+Date: Fri, 2 Jan 2026 11:37:31 -0500
+Subject: [PATCH] Fix a compilation error in TPMLIB_GetPlaintext
+
+Fix a compilation error that newer gcc versions may complain about:
+
+tpm_library.c: In function 'TPMLIB_GetPlaintext':
+tpm_library.c:441:11: error: assignment discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
+  441 |     start = strstr(stream, starttag);
+      |           ^
+At top level:
+cc1: note: unrecognized command-line option '-Wno-self-assign' may have been intended to silence earlier diagnostics
+cc1: all warnings being treated as errors
+
+Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
+
+Upstream: https://github.com/stefanberger/libtpms/commit/fc8820cfaa8b5e17328f731df93911f6ab92443b
+
+Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
+---
+ src/tpm_library.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/src/tpm_library.c b/src/tpm_library.c
+index f48f4fd3..7b2ea687 100644
+--- a/src/tpm_library.c
++++ b/src/tpm_library.c
+@@ -435,7 +435,7 @@ static unsigned char *TPMLIB_GetPlaintext(const char *stream,
+                                           const char *endtag,
+                                           size_t *length)
+ {
+-    char *start, *end;
++    const char *start, *end;
+     unsigned char *plaintext = NULL;
+ 
+     start = strstr(stream, starttag);
+-- 
+2.47.3
+