Fix source_tree variable in dkms build

The source_tree variable in the previous commit had an extra $.
Remove it so that source_tree is expanded properly.  An identical
fix has been applied in the original patch to the stable branch.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2776

scripts/dkms.mkconf

@@ -27,7 +27,7 @@ PRE_BUILD="configure
   --with-config=kernel
   --with-linux=\${kernel_source_dir}
   --with-linux-obj=\${kernel_source_dir}
-  --with-spl=\${$source_tree}/spl-\${PACKAGE_VERSION}/
+  --with-spl=\${source_tree}/spl-\${PACKAGE_VERSION}
   --with-spl-obj=\${dkms_tree}/spl/\${PACKAGE_VERSION}/\${kernelver}/\${arch}
   --with-spl-timeout=300
   \$(