Fix build issues on macOS 10.14

[ghost]

This fixes a build issue on macOS 10.14 (Mojave) where clang complained
that the header files <config.h> and <pinentry-curses.h> could not
be found.

Build errors before this patch: build_output.txt

[fxcoudert]

This is more likely due to newest clang that to macOS version.

[jaredmichaelwilliams]

I was coming to do this as well haha, you beat me to it had to change
<config.h> to "config.h" in these files
./Source/pinentry-0.9.4/assuan/assuan-listen.c ./Source/pinentry-0.9.4/assuan/assuan-buffer.c ./Source/pinentry-0.9.4/assuan/assuan-util.c ./Source/pinentry-0.9.4/pinentry/password-cache.c ./Source/pinentry-0.9.4/pinentry/pinentry.c ./Source/pinentry-0.9.4/pinentry/argparse.c ./Source/pinentry-0.9.4/secmem/secmem.c
and <pinentry-curses.h> to "pinentry-curses.h"
in Source/main.m Source/AppDelegate.m

[kotashiratsuka]

👍