build: Remove src/obj directory from repository

This directory is automatically created by the build process (in the
build target directory, see bitcoin#16588) and doesn't need to be in the
repository nor in the tarballs.

Move associated ignore directive to top-level `.gitignore` file.

Author: laanwj

.gitignore

@@ -64,6 +64,7 @@ libtool
 src/config/pivx-config.h
 src/config/pivx-config.h.in
 src/config/stamp-h1
+src/obj
 share/setup.nsi
 share/qt/Info.plist
 contrib/devtools/split-debug.sh