Fix compilation error with musl in gdb/testsuite/gdb.base/fileio.c

Fix compilation error with musl in gdb/testsuite/gdb.base/fileio.c

Musl is giving warnings about these includes in this way:
warning: #warning redirecting incorrect #include <sys/errno.h> to <errno.h>
warning: #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h>

gdb/testsuite/Changelog:

	* gdb.base/fileio.c: Remove #include of <sys/errno.h>.
	Replace #include of <sys/fcntl.h> by <fcntl.h>.