Doc: Use \c instead of \tt for literal text in platform guides

The Embedded Linux fonts page and the Windows build page mark up the
literal names fontconfig, gRPC, and Protobuf with \tt. The two commands
produce the same output here, and \tt is only needed when other QDoc
commands or macros are nested inside the argument.

This change replaces these four uses with \c.

Pick-to: 6.12
Change-Id: Ie561a48f0e105161725de13a7399c4c50dd97226
Reviewed-by: Topi Reinio <topi.reinio@qt.io>