Be specific about the Maven plugin groupId

Change-Id: I2d170ad046164d62eaf5d50f0cfaacd252a81c1d
Signed-off-by: Shawn O. Pearce <sop@google.com>

pom.xml

diff --git a/pom.xml b/pom.xml
index 11254b2..61d3d91 100644
--- a/pom.xml
+++ b/pom.xml
@@ -316,6 +316,7 @@
         </plugin>
 
         <plugin>
+          <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-compiler-plugin</artifactId>
           <version>2.0.2</version>
         </plugin>
@@ -366,6 +367,7 @@
 
     <plugins>
       <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-compiler-plugin</artifactId>
         <configuration>
           <source>1.6</source>
@@ -375,6 +377,7 @@
       </plugin>
 
       <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-source-plugin</artifactId>
         <executions>
           <execution>