Merge #19: Fix compilation of foo.h: include <string>

c92112a Fix compilation of foo.h: include <string> (Vasil Dimov)

Pull request description:

  std::string is used inside foo.h but it forgot to include the
  corresponding header:

  ```
  [ 53%] Building CXX object CMakeFiles/mptest.dir/src/mp/test/foo.capnp.proxy-server.c++.o
  /usr/bin/c++   -I/usr/local/lib/cmake/CapnProto/../../../include -IBUILD/libmultiprocess/include -ISOURCE/libmultiprocess/include -ISOURCE/libmultiprocess/src -IBUILD/libmultiprocess/src -isystem /usr/local/include -D_THREAD_SAFE -pthread -std=gnu++14 -o CMakeFiles/mptest.dir/src/mp/test/foo.capnp.proxy-server.c++.o -c BUILD/libmultiprocess/src/mp/test/foo.capnp.proxy-server.c++
  In file included from BUILD/libmultiprocess/src/mp/test/foo.capnp.proxy-server.c++:3:
  In file included from BUILD/libmultiprocess/src/mp/test/foo.capnp.proxy-types.h:6:
  In file included from BUILD/libmultiprocess/src/mp/test/foo.capnp.proxy.h:7:
  SOURCE/libmultiprocess/src/mp/test/foo.h:17:17: error: implicit
        instantiation of undefined template 'std::__1::basic_string<char, std::__1::char_traits<char>,
        std::__1::allocator<char> >'
      std::string name;
                  ^
  /usr/include/c++/v1/iosfwd:210:32: note: template is declared here
      class _LIBCPP_TEMPLATE_VIS basic_string;
                                 ^
  1 error generated.
  ...

  $ c++ --version
  FreeBSD clang version 8.0.1 (tags/RELEASE_801/final 366581) (based on LLVM 8.0.1)
  Target: x86_64-unknown-freebsd12.1
  Thread model: posix
  InstalledDir: /usr/bin
  ```

ACKs for top commit:
  ryanofsky:
    Code review ACK c92112a.

Tree-SHA512: bac665541932d826e5c0ed2881860c3d011a041699c6005af5266bb27e82b3961646375e16c329a0b2b7c29214cdd4397a8aaabf7ca2c03efa2413e9cc9b9ca4

Author: ryanofsky

src/mp/test/foo.h

@@ -7,6 +7,7 @@
 
 #include <map>
 #include <memory>
+#include <string>
 #include <vector>
 
 namespace mp {