Expose eigen header. No explaination for why it was hidden (#9)

pybind11.BUILD

diff --git a/pybind11.BUILD b/pybind11.BUILD
index 1a6abea..1367827 100644
--- a/pybind11.BUILD
+++ b/pybind11.BUILD
@@ -22,7 +22,6 @@
 EXCLUDES = [
     # Deprecated file that just emits a warning
     "include/pybind11/common.h",
-    "include/pybind11/eigen.h",
 ]
 
 cc_library(