The changes in #274 (proto3: make strings default to "" instead of NULL)
add a new symbol to the library and add an 'extern' declaration to the
protobuf-c header file.

Since the compiler may generate files that depend on that new
'protobuf_c_empty_string' declaration in protobuf-c.h, we need to bump
the min_header_version value that is written into generated header
files. But since the 'protobuf_c_empty_string' symbol can only be
referenced when generating proto3 syntax files, we only need to use the
stricter min_header_version value when processing a proto3 file.