Provide struct_sock_fprog_sz on Linux, rather than on Linux+glibc.

With `#include <linux/filter.h>` `struct sock_fprog` is available when
linux headers are available. It seems that glibc is just including that header
as an implementation detail, while musl does not.
--- a/libsanitizer/sanitizer_common/sanitizer_platform_limits_posix.cpp	2025-08-08 08:51:45.621434001 +0200
+++ b/libsanitizer/sanitizer_common/sanitizer_platform_limits_posix.cpp	2026-02-25 20:29:28.519492722 +0100
@@ -74,6 +74,7 @@
 #include <sys/vt.h>
 #include <linux/cdrom.h>
 #include <linux/fd.h>
+#include <linux/filter.h>
 #if SANITIZER_ANDROID
 #include <linux/fs.h>
 #endif
@@ -507,6 +508,7 @@ unsigned struct_ElfW_Phdr_sz = sizeof(El
   unsigned struct_seq_event_rec_sz = sizeof(struct seq_event_rec);
   unsigned struct_synth_info_sz = sizeof(struct synth_info);
   unsigned struct_vt_mode_sz = sizeof(struct vt_mode);
+  unsigned struct_sock_fprog_sz = sizeof(struct sock_fprog);
 #endif // SANITIZER_LINUX
 
 #if SANITIZER_GLIBC
@@ -534,7 +536,6 @@ unsigned struct_ElfW_Phdr_sz = sizeof(El
 
   unsigned struct_audio_buf_info_sz = sizeof(struct audio_buf_info);
   unsigned struct_ppp_stats_sz = sizeof(struct ppp_stats);
-  unsigned struct_sock_fprog_sz = sizeof(struct sock_fprog);
 #  endif  // SANITIZER_GLIBC
 
 #  if !SANITIZER_ANDROID && !SANITIZER_APPLE