debuginfod-client.c: Avoid sscanf on mixed-case component of string

debuginfod-client.c: Avoid sscanf on mixed-case component of string

sscanf is used to get the value of x-debuginfod-size from the http
headers.  The format string used assumes that the header field name
is entirely lower case.  However mixed-case field names are possible,
resulting in the value not being read.

Fix this by removing "x-debuginfod-size" from the format string.

Signed-off-by: Aaron Merey <amerey@redhat.com>