audio/rubberband: fix build with libc++ 21
With libc++ 21 audio/rubberband fails to build, with errors similar to:
In file included from ../src/common/mathmisc.cpp:24:
../src/common/mathmisc.h:58:1: error: unknown type name 'size_t'; did you mean 'std::size_t'?
58 | size_t roundUp(size_t value); // to nearest power of two
| ^~~~~~
| 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));
| ^This is because mathmisc.h depended on <stddef.h> being transitively
included, which is no longer the case. Add #include <stddef.h> to fix
the problem.
PR: 292868
Approved by: acm (maintainer)
MFH: 2026Q1
(cherry picked from commit 78992d889cf64999b2ff0b82d6940eb7c9edb12d)