Only define DAV1D_API to dllexport when building dav1d itself

As meson still doesn't allow specifying different cflags between
static and dynamic libraries, this still includes the dllexport
in the static library when built with default_library=both, but
it at least is avoided in static-only builds, and avoids
defining these symbols as dllexport in the callers' translation
units.
21 jobs for master in 6 minutes and 41 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Style
passed #266316
amd64 debian
style-check

00:00:22

 
  Build
passed #266317
amd64 debian
build-debian

00:00:43

passed #266325
debian aarch64
build-debian-aarch64

00:01:40

passed #266326
debian aarch64
build-debian-aarch64-clang-5

00:01:09

passed #266329
debian armv7
build-debian-armv7

00:02:49

passed #266330
debian armv7
build-debian-armv7-clang-5

00:01:11

passed #266318
amd64 debian
build-debian-static

00:00:44

passed #266328
debian aarch64
build-debian-werror

00:00:37

passed #266319
amd64 debian
build-debian32

00:00:34

passed #266327
macos
build-macos

00:00:32

passed #266331
amd64 debian allowed to fail
build-ubuntu-snap

00:01:06

passed #266323
amd64 debian
build-win-arm32

00:00:30

passed #266324
amd64 debian
build-win-arm64

00:00:32

passed #266320
amd64 debian
build-win32

00:00:41

passed #266321
amd64 debian
build-win32-unaligned-stack

00:00:39

passed #266322
amd64 debian
build-win64

00:00:49

 
  Test
passed #266332
amd64 debian
test-debian

00:00:56

passed #266333
amd64 debian
test-debian-asan

00:02:16

passed #266334
amd64 debian
test-debian-msan

00:01:11

passed #266335
amd64 debian
test-debian-ubsan

00:01:26

passed #266336
amd64 debian
test-win64

00:00:48