write/elf: don't write an empty strtab

Always write a strtab for relocatable object files.

Previously we were writing a strtab that had 0 bytes of data.
This gives a linker error:
SHT_STRTAB string table section [index 4] is empty

Also, there must be a strtab, otherwise the error is:
invalid sh_type for string table section [index 0]: expected SHT_STRTAB, but got SHT_NULL

Author: philipc

src/write/elf/object.rs

@@ -600,6 +600,7 @@ impl<'a> Object<'a> {
             writer.reserve_symtab_shndx_section_index();
         }
         writer.reserve_symtab_shndx();
+        writer.require_strtab();
         writer.reserve_strtab_section_index();
         writer.reserve_strtab();