fix multiple patterns used on core_find

'find', the underlying command of core_find, does not support multiple patterns
after -name.  This patch fixes multi-pattern use in c_src.mk by calling core_find
once for each pattern.

plugins/c_src.mk

@@ -60,7 +60,7 @@ clean::
 else
 
 ifeq ($(SOURCES),)
-SOURCES := $(sort $(call core_find,$(C_SRC_DIR)/,*.c *.C *.cc *.cpp))
+SOURCES := $(sort $(foreach pat,*.c *.C *.cc *.cpp,$(call core_find,$(C_SRC_DIR)/,$(pat))))
 endif
 OBJECTS = $(addsuffix .o, $(basename $(SOURCES)))