btrfs-progs: autoconf: set exec_prefix in makefile

Lost in the conversion and breaks the build unless set explicitly.

Reported-by: WorMzy Tykashi <wormzy.tykashi@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/Makefile.in b/Makefile.in
index 93b6a3c..73983cc 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -48,6 +48,7 @@
 TESTS = fsck-tests.sh convert-tests.sh
 
 prefix ?= @prefix@
+exec_prefix = @exec_prefix@
 bindir = @bindir@
 libdir ?= @libdir@
 incdir = @includedir@/btrfs