gcc-plugins: fix build condition of SANCOV plugin

Since commit d677a4d601 ("Makefile: support flag
-fsanitizer-coverage=trace-cmp"), you miss to build the SANCOV
plugin under some circumstances.

  CONFIG_KCOV=y
  CONFIG_KCOV_ENABLE_COMPARISONS=y
  Your compiler does not support -fsanitize-coverage=trace-pc
  Your compiler does not support -fsanitize-coverage=trace-cmp

Under this condition, $(CFLAGS_KCOV) is not empty but contains a
space, so the following ifeq-conditional is false.

    ifeq ($(CFLAGS_KCOV),)

Then, scripts/Makefile.gcc-plugins misses to add sancov_plugin.so to
gcc-plugin-y while the SANCOV plugin is necessary as an alternative
means.

Fixes: d677a4d601 ("Makefile: support flag -fsanitizer-coverage=trace-cmp")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Kees Cook <keescook@chromium.org>

scripts/Makefile.gcc-plugins

@@ -14,7 +14,7 @@ ifdef CONFIG_GCC_PLUGINS
   endif
 
   ifdef CONFIG_GCC_PLUGIN_SANCOV
-    ifeq ($(CFLAGS_KCOV),)
+    ifeq ($(strip $(CFLAGS_KCOV)),)
       # It is needed because of the gcc-plugin.sh and gcc version checks.
       gcc-plugin-$(CONFIG_GCC_PLUGIN_SANCOV)           += sancov_plugin.so