add slf4j.api to core

No idea why it is needed

Author: jan

io.sloeber.core/META-INF/MANIFEST.MF

@@ -23,7 +23,8 @@ Require-Bundle: org.eclipse.cdt.managedbuilder.core,
  org.eclipse.core.variables,
  org.apache.commons.commons-io,
  org.apache.commons.commons-compress,
- com.github.luben.zstd-jni;bundle-version="1.5.5"
+ com.github.luben.zstd-jni;bundle-version="1.5.5",
+ slf4j.api;bundle-version="2.0.9"
 Export-Package: cc.arduino.packages;x-internal:=true,
  cc.arduino.packages.discoverers;x-internal:=true,
  cc.arduino.packages.ssh;x-internal:=true,