Export extract-srcjars.sh to Make

Export extract-srcjars.sh to Make so that it can be used to add
srcjar support to droiddoc rules.

Test: m checkbuild
Change-Id: I203ba6dad731ec9167eb8706bbfb345a484f024b
diff --git a/java/config/makevars.go b/java/config/makevars.go
index 96fa92b..dabf2e7 100644
--- a/java/config/makevars.go
+++ b/java/config/makevars.go
@@ -61,4 +61,5 @@
 	}
 
 	ctx.Strict("SOONG_JAVAC_WRAPPER", "${SoongJavacWrapper}")
+	ctx.Strict("EXTRACT_SRCJARS", "${ExtractSrcJarsCmd}")
 }