libspl/mnttab: make mnttab source filenames consistent

FreeBSD's getextmntent.c is only separate because it has a different
license to mnttab.c, otherwise it would go there too.

Sponsored-by: TrueNAS
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@truenas.com>
Closes #18296

lib/libspl/Makefile.am

@@ -44,7 +44,7 @@ if BUILD_LINUX
 libspl_la_SOURCES += \
 	%D%/os/linux/getexecname.c \
 	%D%/os/linux/gethostid.c \
-	%D%/os/linux/getmntany.c \
+	%D%/os/linux/mnttab.c \
 	%D%/os/linux/zone.c
 endif
 
@@ -52,7 +52,7 @@ if BUILD_FREEBSD
 libspl_la_SOURCES += \
 	%D%/os/freebsd/getexecname.c \
 	%D%/os/freebsd/gethostid.c \
-	%D%/os/freebsd/getmntany.c \
+	%D%/os/freebsd/getextmntent.c \
 	%D%/os/freebsd/mnttab.c \
 	%D%/os/freebsd/zone.c
 endif