mscgen: add 'libwebp' to unbreak build

Fixes broken build since commit 7d580b8339a749acd7f61ca6d1f21687ec9680
("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

pkgs/tools/graphics/mscgen/default.nix

···
-{ stdenv, fetchurl, flex, bison, gd, libpng, libjpeg, freetype, zlib }:
+{ stdenv, fetchurl, flex, bison, gd, libpng, libjpeg, freetype, zlib, libwebp }:
 
 let
   version = "0.20";
···
     sha256 = "3c3481ae0599e1c2d30b7ed54ab45249127533ab2f20e768a0ae58d8551ddc23";
   };
 
-  buildInputs = [ flex bison gd libjpeg libpng freetype zlib ];
+  buildInputs = [ flex bison gd libjpeg libpng freetype zlib libwebp ];
 
   doCheck = true;
   preCheck = ''