Copy header files individually, instead of both files twice.

The osmpbf (1.3.3-1) build for Debian failed on the mipsel architecture.

The Debian package uses parallel builds which caused the following failure:

```
protoc --proto_path=. --cpp_out=. fileformat.proto
protoc --proto_path=. --cpp_out=. osmformat.proto
cp *.pb.h ../include/osmpbf/
cp *.pb.h ../include/osmpbf/
g++ -g -O2 -Wformat -Werror=format-security -D_FORTIFY_SOURCE=2 -Wall -Wextra -pedantic -Wredundant-decls -Wdisabled-optimization -Wctor-dtor-privacy -Wnon-virtual-dtor -Woverloaded-virtual -Wsign-promo -Wno-long-long -fPIC -c -o osmformat.pb.o osmformat.pb.cc
cp: cannot create regular file '../include/osmpbf/fileformat.pb.h': File exists
```

This change fixes the race condition by copying the header files separately,
instead of copying both header files twice.

With this patch applied, the builds succeeded on all architectures where
OpenJDK 7 is available.

Origin: http://anonscm.debian.org/gitweb/?p=pkg-grass/osmpbf.git;a=commitdiff;h=adcf891b95609e375130438dc431e96017bbbbf8
Merge pull request #16 from sebastic/header-copy

Copy header files individually, instead of both files twice.