tools/libxl: compile with -Wmissing-declarations

tools/libxl: compile with -Wmissing-declarations

Since the recent build error caused by mismatch of symbol types due to
missing declarations, build libxl with -Wmissing-declarations.

The patch is mostly straightforward, a one liner in the makefile enables
the flag, a lot of functions in xl_cmdimpl.c needed to be made static
and libxl_paths.c needed to include libxl.h.

The one wart on the patch-set is that flex has a bug where it emits code
with missing declarations for yy(set|get)_column.  This can be worked
around by providing the declarations ourselves regardless.

Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>