Fix printf format specifier in Elf.h

Summary:
The format specifier for printf should be %zu to take into account
32bit machines.

Reviewed By: Orvid

Differential Revision: D9665574

fbshipit-source-id: e7019c1deba80addef36ea8f6ebff48b45eceefa

folly/experimental/symbolizer/Elf.h

@@ -246,7 +246,7 @@ class ElfFile {
       snprintf(
           msg,
           sizeof(msg),
-          "Offset (%lu + %lu) is not contained within our mmapped"
+          "Offset (%zu + %zu) is not contained within our mmapped"
           " file (%s) of length %zu",
           offset,
           sizeof(T),