Fix compile warnings about using platform encoding

[jlmuir]

Set the project.build.sourceEncoding property to UTF-8 to eliminate the following compile phase warnings:

$ mvn compile
[INFO] Scanning for projects...
[INFO]
[INFO] -------< com.redhat.jboss.maven:el-profile-activator-extension >--------
[INFO] Building El Profile Activator Maven Extension 1.0.0-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ el-profile-activator-extension ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /tmp/el-profile-activator-extension/src/main/resources
[INFO]
[INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ el-profile-activator-extension ---
[INFO] Changes detected - recompiling the module!
[WARNING] File encoding has not been set, using platform encoding UTF-8, i.e. build is platform dependent!
[INFO] Compiling 1 source file to /tmp/el-profile-activator-extension/target/classes
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.201 s
[INFO] Finished at: 2018-10-24T15:12:47-05:00
[INFO] ------------------------------------------------------------------------