Updated volk submodule to latest maint branch

Submodule volk f28a8be..afd7878:
  > volk: using hypot instead of cabsf.
  > build: fix static linking for volk applications
  > profile: catch exception while compiling regexes
  > qa: remove obsolete check that results is a vector
  > prefs: replace strcpy with a bounded strncpy
  > profile: fix potential buffer overrun
  > profile: clarify that --update can be used with -R
  > tmpl: delete include for removed gcc_x86_cpuid header

Author: jmcorgan