scratch_buffer: Fix dependencies

[Bruno Haible]

lib/malloc/scratch_buffer.h states that for
  scratch_buffer_grow
  scratch_buffer_grow_preserve
  scratch_buffer_set_array_size
  scratch_buffer_dupfree
errno gets set upon failure. Looking at the code, this requires
  for scratch_buffer_grow: requires malloc-posix,
  for scratch_buffer_grow_preserve: requires malloc-posix, realloc-posix, 
free-posix,
  for scratch_buffer_set_array_size: requires malloc-posix,
  for scratch_buffer_dupfree: requires malloc-posix, realloc-posix.


2021-01-30  Bruno Haible  <bruno@clisp.org>

        scratch_buffer: Fix dependencies.
        * modules/scratch_buffer (Depends-on): Add malloc-posix, realloc-posix,
        free-posix.

diff --git a/modules/scratch_buffer b/modules/scratch_buffer
index 7eedae7..cf83ab5 100644
--- a/modules/scratch_buffer
+++ b/modules/scratch_buffer
@@ -14,6 +14,9 @@ c99
 libc-config
 stdbool
 stddef
+malloc-posix
+realloc-posix
+free-posix
 
 configure.ac: