git: Update Linux and macOS build instructions to use Ninja

dist/compiling/linux.md

@@ -9,9 +9,10 @@ On Linux, ImHex is built through regular GCC (or optionally Clang).
 cd ImHex
 mkdir -p build
 cd build
-CC=gcc-12 CXX=g++-12 cmake                    \
+CC=gcc-12 CXX=g++-12                          \
+cmake -G "Ninja"                              \
     -DCMAKE_BUILD_TYPE=Release                \
-    -DCMAKE_INSTALL_PREFIX="/usr" 	          \
+    -DCMAKE_INSTALL_PREFIX="/usr"             \
     -DCMAKE_C_COMPILER_LAUNCHER=ccache        \
     -DCMAKE_CXX_COMPILER_LAUNCHER=ccache      \
     -DCMAKE_C_FLAGS="-fuse-ld=lld"            \