Bug 578092 - PGPSignatureVerifier.getSignatures and
PGPPublicKeyStore.readPublicKeys have questionable uses of
String.getBytes()

Use StandardCharsets.US_ASCII to encode the bytes.

Change-Id: I790bd735309e3ddf6f0294751f0b6ef7120f5564
Signed-off-by: Ed Merks <ed.merks@gmail.com>
Reviewed-on: https://git.eclipse.org/r/c/equinox/rt.equinox.p2/+/189671
Tested-by: Equinox Bot <equinox-bot@eclipse.org>