networkmanager: Fix builds with musl

On musl, build would fail with:

| In file included from
../NetworkManager-1.22.10/shared/systemd/src/basic/env-util.c:11:
|
../NetworkManager-1.22.10/shared/systemd/src/basic/alloc-util.h:100:35:
error: static declaration of 'reallocarray' follows non-static
declaration
|   100 | _alloc_(2, 3) static inline void *reallocarray(void *p, size_t
need, size_t size) {
|       |                                   ^~~~~~~~~~~~
| In file included from
../NetworkManager-1.22.10/shared/nm-default.h:184,
|                  from
../NetworkManager-1.22.10/shared/systemd/sd-adapt-shared/nm-sd-adapt-shared.h:9,
|                  from
../NetworkManager-1.22.10/shared/systemd/src/basic/env-util.c:3:
|
[...]/build-linux/tmp/work/aarch64-poky-linux-musl/networkmanager/1.22.10-r0/recipe-sysroot/usr/include/stdlib.h:148:7:
note: previous declaration of 'reallocarray' with type 'void *(void *,
size_t,  size_t)' {aka 'void *(void *, long unsigned
 int,  long unsigned int)'}
|   148 | void *reallocarray (void *, size_t, size_t);
|       |       ^~~~~~~~~~~~

This is because musl does't provide reallocarray as part of malloc.h but
only as part of stdlib.h. Configure script was modified to check for its
definition in stdlib.h as well (backported patch).

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>
Signed-off-by: Bernhard Rosenkraenzer <bernhard.rosenkraenzer.ext@huawei.com> [adapt filename in .bbappend to match the actual patch filename]