Fix SLF4J binding for annotation processors (helidon-io#7706)

Add slf4j-api as a dependency to inject/tools.
This ensures when the compiler plugin resolves the processor transitive dependencies the resolved version of slf4j-api is the managed version.
Otherwise we may end-up with different versions between slf4j-api and slf4j-jdk14 and a failed binding with the warning.

Fixes helidon-io#7129

Author: romain-grecourt

inject/tools/pom.xml

@@ -67,6 +67,10 @@
         </dependency>
         <dependency>
             <!-- required as handlebars use slf4j -->
+            <groupId>org.slf4j</groupId>
+            <artifactId>slf4j-api</artifactId>
+        </dependency>
+        <dependency>
             <groupId>org.slf4j</groupId>
             <artifactId>slf4j-jdk14</artifactId>
         </dependency>