Evaluate the analytic add-back of the singular integrals in quad precision

Author: CharlesCNorton

src/vmecpp/cpp/vmecpp/free_boundary/singular_integrals/BUILD.bazel

@@ -5,6 +5,7 @@ cc_library(
     name = "singular_integrals",
     srcs = ["singular_integrals.cc"],
     hdrs = ["singular_integrals.h"],
+    linkopts = ["-lquadmath"],
     visibility = ["//visibility:public"],
     deps = [
         "//vmecpp/common/util:util",