Fix header dependency chains in BUILD.gn file

Previous commits adding the libjpeg_turbo_unittests target did not
declare some header dependencies as "public_deps", breaking builds
that relied upon the "libjpeg" target.

This commit fixes the broken header file chain and has been verified
using: gn check <build-directory>

Bug: 993876
Change-Id: I616628fb384c47063ac9fcecc86b65dc155aab6c
diff --git a/BUILD.gn b/BUILD.gn
index d733cc1..548aab5 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -192,7 +192,7 @@
   include_dirs = [ "." ]
 }
 
-source_set("libjpeg_sources") {
+static_library("libjpeg") {
   sources = [
     "jcapimin.c",
     "jcapistd.c",
@@ -250,12 +250,7 @@
     "jutils.c",
     "jversion.h",
   ]
-}
 
-static_library("libjpeg") {
-  deps = [
-    ":libjpeg_sources"
-  ]
   defines = [
     "WITH_SIMD",
     "NO_GETENV",
@@ -289,10 +284,6 @@
     "wrppm.c",
   ]
 
-  deps = [
-    ":libjpeg_sources"
-  ]
-
   defines = [
     "WITH_SIMD",
     "BMP_SUPPORTED",
@@ -303,7 +294,7 @@
 
   public_configs = [ ":libjpeg_config" ]
   public_deps = [
-    ":libjpeg_headers",
+    ":libjpeg",
   ]
 
   # MemorySanitizer doesn't support assembly code, so keep it disabled in