Introduce nvim namespace: Fix unmasked strings.h issue.

Problem:  Now that nvim/strings.h is correctly namespaced, an issue
          that had been masked until now arises:

          When compiling, we get a lot of errors because of everywhere
          the functions in nvim/strings.h are used, there's no include
          to import them.

          But, how could this compile and work previously, then? It
          turns out that:
          - In every such case, we are also including vim.h, which in
            turn includes os_unix_defs.h.
          - os_unix_defs.h includes <string.h> and also <strings.h> in
            some systems (e.g. OSX).
          - Build had been modified previously to (even when importing
            system headers), prefer equally-named local ones. That was
            in fact done as a previous attempt to solve the same issue
            we are trying to solve another way now.

          So, we were including our "strings.h" as a side-effect of
          including <strings.h> through "vim.h" --> "os_unix_defs.h".

Solution: Correctly include "nvim/strings.h" in every file needing it.

src/nvim/syntax.c

@@ -36,6 +36,7 @@
 #include "nvim/path.h"
 #include "nvim/regexp.h"
 #include "nvim/screen.h"
+#include "nvim/strings.h"
 #include "nvim/term.h"
 #include "nvim/ui.h"
 #include "nvim/os/os.h"