gdbserver: include gdbsupport/common-inferior.h in inferiors.c

gdbserver: include gdbsupport/common-inferior.h in inferiors.c

So that the definitions of get_inferior_cwd/set_inferior_cwd see their
declarations.

      CXX    inferiors.o
    /home/smarchi/src/binutils-gdb/gdb/gdbserver/inferiors.c: In function ‘const char* get_inferior_cwd()’:
    /home/smarchi/src/binutils-gdb/gdb/gdbserver/inferiors.c:228:1: error: no previous declaration for ‘const char* get_inferior_cwd()’ [-Werror=missing-declarations]
     get_inferior_cwd ()
     ^~~~~~~~~~~~~~~~
    /home/smarchi/src/binutils-gdb/gdb/gdbserver/inferiors.c: In function ‘void set_inferior_cwd(const char*)’:
    /home/smarchi/src/binutils-gdb/gdb/gdbserver/inferiors.c:236:1: error: no previous declaration for ‘void set_inferior_cwd(const char*)’ [-Werror=missing-declarations]
     set_inferior_cwd (const char *cwd)
     ^~~~~~~~~~~~~~~~

gdb/gdbserver/ChangeLog:

	* inferiors.c: Include gdbsupport/common-inferior.h.

Change-Id: Iae5ccb3e1dc37ce79f03f08465f603a0411e7af0