doc: ensure sdcc build-deps do not install graphicsmagick

On Debian-derived distros, only install the build-deps which are
required to build sdcc for ngdevkit. This prevents installing
conflicting package graphicsmagick-imagemagick-compat.

Resolves: #4

README-mingw.md

@@ -50,7 +50,7 @@ tools:
     sudo apt-get install gcc curl unzip imagemagick
     GCC_VERSION_PKG=$(apt-cache depends gcc | awk '/Depends.*gcc/ {print $2}')
     sudo apt-get build-dep $GCC_VERSION_PKG
-    sudo apt-get build-dep sdcc
+    sudo apt-get build-dep --arch-only sdcc
     sudo apt-get install libsdl2-dev
     sudo apt-get install python-pygame
     sudo apt-get install automake

README.md

@@ -41,7 +41,7 @@ On a Debian-derived distro, this can be done with:
     apt-get install gcc curl imagemagick
     GCC_VERSION_PKG=$(apt-cache depends gcc | awk '/Depends.*gcc/ {print $2}')
     apt-get build-dep $GCC_VERSION_PKG
-    apt-get build-dep sdcc
+    apt-get build-dep --arch-only sdcc
     apt-get install libsdl2-dev
     apt-get install python-pygame