add missing package to build windows distribution

Author: frederichecht

source/introduction/installation.rst

@@ -614,7 +614,7 @@ Repeatedly run the following command until it says there are no further updates.
       pacman -S autoconf make automake-wrapper bison git \
         mingw-w64-x86_64-freeglut mingw-w64-x86_64-toolchain \
         mingw-w64-x86_64-openblas patch python perl pkg-config pkgfile \
-        rebase tar time tzcode unzip which  \
+        rebase tar time tzcode unzip which mingw-w64-x86_64-gsl \
         --ignore mingw-w64-x86_64-gcc-ada --ignore mingw-w64-x86_64-gcc-objc \
         --ignore mingw-w64-x86_64-gdb mingw-w64-x86_64-cmake --noconfirm
 
@@ -631,7 +631,7 @@ Repeatedly run the following command until it says there are no further updates.
         mingw-w64-i686-openblas mintty msys2-keyring msys2-launcher-git \
         msys2-runtime ncurses pacman pacman-mirrors pactoys-git patch pax-git \
         perl pkg-config pkgfile rebase sed tar tftp-hpa time tzcode unzip \
-        util-linux which
+        util-linux which 
 
 6. Open a ``MingW64 terminal`` (or ``MingW32`` for old 32 bit **FreeFEM** version) and compile the **FreeFEM** source