qemu-build.sh: add python3-venv dependency

The latest 8.1 QEMU requires this be in the docker image now

Tested:
- Confirmed I can build latest openbmc/qemu with this

Change-Id: Ib2163840402f2034d600f419d83dbef641362cf6
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/qemu-build.sh b/qemu-build.sh
index c17b654..b2d0224 100755
--- a/qemu-build.sh
+++ b/qemu-build.sh
@@ -127,6 +127,7 @@
     libslirp-dev \
     make \
     ninja-build \
+    python3-venv \
     python3-yaml \
     iputils-ping