Decrypt: Move ENABLE_NSS3 to poppler-config.h

Since ENABLE_NSS3 is used in a header file now, move its definition
to poppler-config.h.
6 jobs for implement-crypto-using-NSS in 3 minutes and 11 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #375516
build

00:03:11

passed #375521
build_android

00:01:02

passed #375517
build_clang_libcpp

00:03:07

passed #375520
build_clazy

00:03:10

passed #375519
build_mingw64_fedora

00:02:52

passed #375518
build_ubuntu_16_04

00:01:29