libsecret-1 dependency should be optional

[Nexion]

Expected Behavior

proton-bridge should be able to build without libsecret and use pass as a credential manager.

Current Behavior

# pkg-config --cflags  -- libsecret-1
Package libsecret-1 was not found in the pkg-config search path.
Perhaps you should add the directory containing `libsecret-1.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libsecret-1', required by 'virtual:world', not found
pkg-config: exit status 1
make: *** [Makefile:78: build-nogui] Error 2

[exander77]

I think this as well. I had to comment that out for Android as the only pass is available there and no libsecret.

[LBeernaertProton]

We currently don't see this as a priority, but we welcome any PR that would allow this to be possible.