java-utils: Remove blank spaces before reading package name

Closes https://github.com/mate-desktop/engrampa/issues/291

Test:

touch test.txt
zip test.zip test.txt
mv test.zip test.jar
cat << EOF > HelloWorld.java
package org.mate.tests;

class HelloWorld
{
    public static void main(String args[])
    {
        System.out.println("Hello, World");
    }
}
EOF
engrampa -a test.jar HelloWorld.java
jar tf test.jar

Output:

test.txt
org/mate/tests/HelloWorld.java

0 files changed, 0 insertions, 0 deletions