mscgen: add 'libwebp' to unbreak build

Fixes broken build since commit 7d580b83
("gd: 2.1.1 -> 2.2.1"):

  $ nix-build -A mscgen
  [...]
  gcc  -g -O2   -o mscgen mscgen-adraw.o mscgen-cmdparse.o mscgen-main.o
  mscgen-svg_out.o mscgen-language.o mscgen-msc.o mscgen-ps_out.o
  mscgen-utf8.o mscgen-gd_out.o mscgen-safe.o mscgen-lexer.o
  mscgen-null_out.o mscgen-usage.o -lm -lgd -lm
  -L/nix/store/HASH-zlib-1.2.8/lib -lz
  -L/nix/store/HASH-libpng-apng-1.6.21/lib
  -lpng16 -L/nix/store/HASH-freetype-2.6.2/lib
  -lfreetype
  -L/nix/store/HASH-fontconfig-2.11.1-lib/lib
  -L/nix/store/HASH-freetype-2.6.2/lib
  -lfontconfig -lfreetype -ljpeg -lwebp -lgd -lm
  /nix/store/HASH-binutils-2.26/bin/ld: cannot find -lwebp
  collect2: error: ld returned 1 exit status