Fix 64-bit build.

Fix 64-bit build.

A warning about signed vs unsigned comparison was converted
into an error here :

 ...
 struct stat st;
 if (st.st_size > sizeof(prop_area) {
 ...

st_size is either an off64_t, which is a signed type. It's
worth investigating why this didn't trigger a warning on 32 bit,
where it's signed as well.

Change-Id: Ib2622bd5c444ddcfa7fb2141f00332cbb4a0818b