[build] Add missing header to build (regexp/special-case.h)

This was missed in a file move

Bug: chromium:991547
Change-Id: I4442f2e4e7acdb91b16151714b87e229bce1a409
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1824945
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Auto-Submit: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63968}

BUILD.gn

@@ -2791,13 +2791,13 @@ v8_source_set("v8_base_without_compiler") {
     "src/regexp/regexp-nodes.h",
     "src/regexp/regexp-parser.cc",
     "src/regexp/regexp-parser.h",
-    "src/regexp/regexp-special-case.h",
     "src/regexp/regexp-stack.cc",
     "src/regexp/regexp-stack.h",
     "src/regexp/regexp-utils.cc",
     "src/regexp/regexp-utils.h",
     "src/regexp/regexp.cc",
     "src/regexp/regexp.h",
+    "src/regexp/special-case.h",
     "src/roots/roots-inl.h",
     "src/roots/roots.cc",
     "src/roots/roots.h",