Fix linker section pattern to match pte_to_header.py output

pte_to_header.py generates __attribute__((section("network_model_sec")))
(no leading dot), but the linker snippet used *(.network_model_sec) which
doesn't match.  The model ended up in MODEL_DDR anyway because the linker
placed the orphan into the identically-named output section, but this
produced a warning.  Drop the leading dots so the pattern matches directly.

Co-authored-by: Claude <noreply@anthropic.com>

Author: psiddh

zephyr/samples/mv2-ethosu/model_section.ld.in

@@ -8,6 +8,6 @@
 SECTION_DATA_PROLOGUE(@ET_PTE_SECTION@,,)
 {
     . = ALIGN(16);
-    *(.@ET_PTE_SECTION@)
-    *(.@ET_PTE_SECTION@.*)
+    *(@ET_PTE_SECTION@)
+    *(@ET_PTE_SECTION@.*)
 } GROUP_DATA_LINK_IN(MODEL_DDR, MODEL_DDR)