[libcxx22] size_t for math/labplot

<cmath> no longer pulls in the definition for size_t. Not sure if the
below is the best fix, but it works...

/usr/ports/pobj/labplot-2.12.1/labplot-2.12.1/src/backend/nsl/nsl_peak.h:16:1: error: unknown type name 'size_t'; did you mean 'std::size_t'?
   16 | size_t* nsl_peak_detect(T* data, size_t n, size_t& np, T height = -INFINITY, size_t distance = 0);
      | ^~~~~~
      | std::size_t
/usr/include/c++/v1/__cstddef/size_t.h:20:7: note: 'std::size_t' declared here
   20 | using size_t = decltype(sizeof(int));
      |       ^

Index: Makefile
===================================================================
RCS file: /cvs/ports/math/labplot/Makefile,v
diff -u -p -r1.22 Makefile
--- Makefile	24 Jul 2026 17:11:06 -0000	1.22
+++ Makefile	21 Aug 2026 05:15:52 -0000
@@ -1,7 +1,7 @@
 COMMENT =	data visualization and analysis software
 
 DISTNAME =	labplot-2.12.1
-REVISION =	1
+REVISION =	2
 
 CATEGORIES =	math x11
 
Index: patches/patch-src_backend_nsl_nsl_peak_h
===================================================================
RCS file: patches/patch-src_backend_nsl_nsl_peak_h
diff -N patches/patch-src_backend_nsl_nsl_peak_h
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ patches/patch-src_backend_nsl_nsl_peak_h	21 Aug 2026 05:15:19 -0000
@@ -0,0 +1,12 @@
+Index: src/backend/nsl/nsl_peak.h
+--- src/backend/nsl/nsl_peak.h.orig
++++ src/backend/nsl/nsl_peak.h
+@@ -12,6 +12,8 @@
+ 
+ #include <cmath>
+ 
++#include <stddef.h>
++
+ template<typename T>
+ size_t* nsl_peak_detect(T* data, size_t n, size_t& np, T height = -INFINITY, size_t distance = 0);
+