Add a missing include of stdlib.h

Since 7ab4c928, osdep.h contains
calls to malloc/free.

This fixes building with MSVC targeting WinRT.
15 jobs for master in 9 minutes and 35 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #469942
aarch64 docker
build-debian-aarch64

00:08:33

passed #469941
amd64 docker
build-debian-amd64

00:01:28

passed #469945
macos
build-macos

00:02:14

passed #469943
amd64 docker
build-win32

00:01:45

passed #469944
amd64 docker
build-win64

00:01:59

 
  Test
passed #469947
aarch64 docker
test-debian-aarch64

00:00:25

passed #469946
amd64 docker
test-debian-amd64

00:00:17

passed #469950
macos
test-macos

00:00:11

passed #469948
amd64 docker
test-win32

00:00:20

passed #469949
amd64 docker
test-win64

00:00:21

 
  Release
passed #469952
aarch64 docker manual
release-debian-aarch64

00:00:21

passed #469951
amd64 docker manual
release-debian-amd64

00:00:17

passed #469955
macos manual
release-macos

00:00:03

passed #469953
amd64 docker manual
release-win32

00:00:17

passed #469954
amd64 docker manual
release-win64

00:00:16