Document the need to have libncurses5 installed

The prebuilt version of clang requires `libtinfo.so.5` to be present,
which is provided by ncurses5.

Signed-off-by: Philipp Wagner <phw@lowrisc.org>

apt-requirements.txt

diff --git a/apt-requirements.txt b/apt-requirements.txt
index 882fb33..802cf60 100644
--- a/apt-requirements.txt
+++ b/apt-requirements.txt
@@ -22,6 +22,8 @@
 libelf-dev
 libftdi1-2
 libftdi1-dev
+# A requirement of the prebuilt clang toolchain.
+libncurses5
 libssl-dev
 libusb-1.0-0
 lsb-release