Move kmsgrab dependencies from optdepends to depends

kmsgrab is the most fully featured capture backend for current
versions of Sunshine, so it should be built by default.

In addition to zero-copy capture and HDR support, it is the *only*
capture backend that can handle non-wlroots Wayland capture.

docker/archlinux.dockerfile

@@ -34,7 +34,7 @@ ENV COMMIT=${COMMIT}
 
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]
 # install dependencies
-# cuda, libcap, and libdrm are optional dependencies for PKGBUILD
+# cuda is an optional build-time dependency for PKGBUILD
 RUN <<_DEPS
 #!/bin/bash
 set -e
@@ -43,8 +43,6 @@ pacman -Syu --disable-download-timeout --needed --noconfirm \
   cmake \
   cuda \
   git \
-  libcap \
-  libdrm \
   namcap
 _DEPS